Hi, I’m Asami. My project “adding QEMU/AArch64 support to coreboot” is making good progress. I’ve almost done to write porting code and I’m now writing a new tool to make a FIT payload for QEMU/AArch64. Here is my CL. In this article, I’m going to talk about how to run C code in the bootblock stage.
The way to run C code before DRAM has been initialized is various in the coreboot project. The most famous and used in the x86 system is known as the Cache-As-Ram (CAR). CAR can be set up by 1. enable CPU cache 2. enable the ‘no eviction’ mode 3. change cache mode from write-through to write-back. ‘No-eviction’ means that the CPU doesn’t write any data to DRAM as long as the size of data is less than the CPU cache. Then, you can get all the data from the CPU cache. The implementation for CAR is src/cpu/intel/car/non-evict/cache_as_ram.S.
Another way to run C code especially implemented in ARM system is relocating the bootblock code to SRAM. System on a Chip (SoC) has an ARM CPU and often includes SRAM as a cache. For example, QEMU VExpress machine has SRAM is located at 0x48000000 that we can know in qemu/hw/arm/vexpress.c#L120.
However, what should the system that doesn’t have SRAM do? My target machine, QEMU virt machine, doesn’t have SRAM according to the implementation of qemu/hw/arm/virt.c. In this case, should we initialize DRAM earlier?
The answer is No but it’s only for my project. Because QEMU is not actual hardware so DRAM already works. I just need to relocate the bootblock code to DRAM directly. I believe that other ARM systems that have no SRAM should initialize DRAM earlier.
Hello again! It’s been three weeks since my last post (well, one of those weeks I was on vacation, so more like two weeks), so there are many many updates to write about. If you recall from my last post, Coverity has been down for maintenance, so I started looking around for other things to do. Here is a list of some of the highlights:
Coverity isn’t the only static analyzer we use – coreboot also runs nightly scans using the Clang Static Analyzer, an open source tool from the LLVM project. The results from these scans aren’t as detailed as Coverity and unfortunately contain many duplicates and false positives, but I was able to submit patches for some of the issues. Usability of the analyzer is also still somewhat limited, since we have no way of recording fixes for past issues or ignoring false positives. (A framework like CodeChecker might be able to help with this.)
coreboot will (hopefully) soon be (almost) VLA free! Variable length arrays (VLAs) were a feature added in C99 that allows the length of an array to be determined at run time, rather than compile time. While convenient, this is also very dangerous, since it allows use of an unlimited amount of stack memory, potentially leading to stack overflow. This is especially dangerous in coreboot, which often has very little stack space to begin with. Fortunately, almost all of the VLAs in coreboot can be replaced with small fixed-sized buffers, aside from one tricky exception that will require a larger rewrite. This patch was inspired by a similar one for Linux, and is currently under review.
Similar to the above, coreboot will also soon be free of implicit fall throughs! Switches in C have the unfortunate property of each case statement implicitly falling through to the next one, which has led to untold number of bugs over the decades and at least 38 in coreboot itself (by Coverity’s count). GCC however recently added the -Wimplicit-fallthrough warning, which we can use to banish them for good. As of this patch, all accidental fall throughs have been eliminated, with the intentional ones marked with the /* fall through */ comment. Hopefully this is the last we see of this type of bug.
coreboot now has a single implementation of <stdint.h>. Previously, each of the six supported architectures (arm, arm64, mips, ppc64, riscv, and x86) had their own custom headers with arch-specific quirks, but now they’ve all been tidied up and merged together. This should make implementing further standard library headers easier to do (such as <inttypes.h>).
Finally, many coreboot utilities and tools such as flashrom, coreinfo, nvramtool, inteltool, ifdtool, and cbmem have stricter compiler warnings, mostly from -Wmissing-prototypes and -Wextra. Fixing and adding compiler warnings like these are very easy to do, and make great first time contributions (-Wconversion and -Wsign-compare in particular can always use more work).
That’s it for this week! Thankfully Coverity is back online now, so expect more regular blog posts in the future.
Hello again, I’m Asami. I’ve just finished 4 weeks as a GSoC student. I’m currently debugging the implementation of my main project, which is adding QEMU/AArch64 support. I can see nothing output right now when I start a QEMU with the coreboot.rom that has my implementation. It means there is something wrong before a hardware initialization has finished. In this article, I’m going to talk about what I found while debugging the bootblock for ARMv8.
Code Path of Bootblock Stage
The bootblock is executed just after CPU reset and it is almost written by assembly language. The main task is to set up a C-environment. The basic code path for ARMv8 from the beginning the bootblock to the romstage is:
_start() at src/arch/arm64/armv8/bootblock.S
arm64_init_cpu() at src/arch/arm64/armv8/cpu.S
main() at srclib/bootblock.c
run_romstage() at src/lib/prog_loaders.c
prog_run() at src/lib/prog_ops.c
arch_prog_run() at src/arch/arm64/boot.c
main() at src/arch/arm64/romstage.c // The entry point of the romstage
You can use your custom _start function instead of the common _start function by CONFIG_BOOTBLOCK_CUSTOM=y and adding bootblock-y += bootblock_custom.S which is your custom assembly file.
The Reason Why Execution Stopped inside arm64_init_cpu()
I found that an execution stopped inside the arm64_init_cpu function for some reason. The line that has some problem is mrs x22, sctlr_el3 . MRS instruction can read a system control register and store the value into a general purpose register. So this line means to store the value of SCTLR_EL3 into the X22 register.
According to the “ARM Architecture Reference Manual ARMv8, for ARMv8-A architecture profile”, the purpose of SCTLR_EL3 is
Provides top level control of the system, including its memory system, at EL3. This register is part of the Other system control registers functional group.
Also, SCTLR_EL3 is accessible only from EL3 mode. EL3 is the highest privileged mode that a low-level firmware, including the secure monitor, works on it.
Next, I checked the current mode via mrs x0, CurrentEL. CurrentEL is a register that holds the current exception level. The result of CurrentEL was 0x04, which means the program works on EL1 mode. EL1 is the mode that an operating system kernel typically described as privileged. I didn’t have the right to access SCTLR_EL3. That’s why an execution stopped.
Ideas to Solve EL3 Issue
I considered 2 solutions:
Use only EL1 registers
Run QEMU in EL3
Firstly, I tried to use only EL1 registers. I replaced arm64_init_cpu with arm64_init_cpu_el1 that is a new function I created. Then I replaced SCTLR_EL3 with SCTLR_EL1 and TLBI ALLE3 with TLBI VMALLE1. It seems to work well but still, there was nothing output.
Secondly, I tried to run QEMU in EL3 that is enabled by -machine flag. QEMU can work on EL2 with -machine virtualization=on and EL3 with -machine secure=on to enable EL3. The following command works well for me.
Hello again! If you recall from my last post, the schedule this week is to fix the issues in northbridge/via and southbridge. However, Coverity is going through a major internal upgrade, and so the issue tracker has been offline all week. Luckily though I was able to fix most of these issues last week, so assuming the upgrade finishes soon I won’t be behind schedule. In the mean time, I decided to try flashing coreboot onto my T500, since the last component I was waiting for arrived last week. Here is a little mini-guide to my (sometimes harrowing) flashing experience.
5V 2A power adapter for the BBB
Pomona 5252 Test Clip
Atheros AR9462 Wireless Card
Updating the EC
It is generally recommended to update the embedded controller firmware before flashing coreboot, which can only be done during a Lenovo BIOS update. (Unlike Chromebooks, ThinkPads unfortunately do not have open source EC’s.) I was able to find a copy of the latest BIOS on the Lenovo EOL Portal, and attempted to perform an update … which froze and crashed halfway through. Uh oh. This is OK, as long as I don’t restart the computer I can just try flashing it again, right? Wrong! The next time I tried it Windows ran into a fatal error and decided to force a restart for me (gah!). Upon booting it up again, I was met with absolutely nothing, because the screen wouldn’t even turn on. More than a little concerned that I had bricked it, I searched through online forums until I stumbled across the Crisis Recovery tool. Apparently, old ThinkPads have a method to force-update the BIOS from an external USB stick or floppy (if you have one of those lying around). The recovery tool had to be run in Windows XP Service Pack 3 emulation mode, and seemed to format the USB correctly. My ThinkPad wasn’t so impressed, and obstinately refused to recognize the stick. As a last hope, I asked around on IRC what to do, and Nico Huber informed me that the ThinkPad was likely not dead, and that I could just proceed with flashing coreboot anyway. Well, here goes nothing.
So we’re going to flash coreboot, but what options do I pick when compiling it? I scoured around the internet to find tutorials for flashing coreboot onto a T500 and other related ThinkPads, but they all recommended different options, sometimes contradictory. Hmmmm. Once again going back to IRC, Angel Pons helped me configure a very minimal build.
General setup ---> [*] Use CMOS for configuration values
---> [*] Allow use of binary-only repository
Mainboard ---> Mainboard vendor ---> Lenovo
---> Mainboard model ---> ThinkPad T500
Devices ---> Display ---> Linear "high-resolution" framebuffer
Now, the T500 is a very special laptop, in that it can run coreboot without any binary blobs at all. However, I decided to enable microcode updates anyway, since they provide important stability improvements (like not crashing). This laptop also comes with an Intel ME which can be completely wiped, but I decided to leave that for later. (Now that I know coreboot works, there will be a follow-up post in several weeks when I do that.)
Disassembly and Flashing
Like most laptops, the flash IC of the T500 is locked from the factory, and requires an initial external flash to install coreboot (afterwards, subsequent flashes can be done internally). This requires disassembling the laptop to access the SOIC-16, which is buried in the bowels of the T500 case and requires a complete tear-down to access. The Libreboot T500 page gives you a feel for the amount of work required to extract the motherboard, which along with the hardware maintenance manual I referred to extensively.
With the motherboard extracted from the case, the next step is to attach the Pomona 5252 to the SOIC-16 and jumper it to the BBB, which was all made very easy by this X200 guide. Somewhat blithely following the previous guide, I set up an old ATX PSU to provide 3.3v to the flash chip. However, whenever I connected it to the test clip, it would always power itself off. Strange. Going back to IRC, Nico informed me that this is in fact A VERY BAD AND DANGEROUS THING TO DO. THE INTERNET IS LYING – DO NOT USE AN ATX PSU, YOU COULD FRY YOUR MOTHERBOARD! Oops. After puzzling over how to provide enough power to the chip without the PSU, Patrick Rudolph chimed in that a) the T500 motherboard is basically indestructible (whew!), and b) the flasher itself should be able to provide enough power. Hooking the 3.3v cable into the BBB instead, I tried reading the flash chip.
$ flashrom -p linux_spi:dev=/dev/spidev1.0,spispeed=512
(a bunch of output that I forgot to write down)
It works! Even with a bricked Lenovo BIOS, it is still recommended to keep a backup, so next we read the old factory ROM.
Note that because I left the ME as-is, it is important to only flash the BIOS region, not the entire chip.
Reassembly and Testing
Sadly, no instant gratification here – I had to reassemble half the laptop before I could test booting it up. However, after doing so and gingerly pressing the power button, I was greeted by the lovely SeaBIOS boot menu. It actually worked! Huzzah! Finishing reassembly, I replaced the factory Intel wireless card with an Atheros AR9462, which can run without any binary firmware. After installing Debian, I now have a laptop running completely free and open source software, all the way from the BIOS up (well, except for the ME, but I’ll fix that later).
For the final icing on the cake, here is a fresh board status report for the T500. Many thanks to everyone who helped me in this process.
Hello everyone. I am Asami and a student for this year’s GSoC project.My project is adding a new mainboard QEMU/AArch64 to make it easier for coreboot developers to support new boards for ARMv8. I’ve already written a small patch to enable building a sample program with libpayload for ARM architecture. Also, I’ve read the implementation of coreboot (main code path) for ARMv7 and QEMU (qemu/hw/arm/vexpress.c). Now, I just created a new CL for my main project and I started to read the implementation of the target machine of AArch64 (qemu/hw/arm/virt.c).
In this article, I’m going to talk about my mistakes when I developed coreboot. I hope it helps for beginners of coreboot development. The target board is QEMU/ARM and the CPU is ARMv7.
“ERROR: Ramstage region _postram_cbfs_cache overlapped by: fallback/payload”
I faced this error when I built coreboot.rom for QEMU/ARM with the coreinfo which is a small informational payload for coreboot. The cause is that the coreinfo doesn’t support ARM architecture and then the payload is compiled as a 32-bit x86.
Make sure that your payload is your target architecture. You need to use other executable files instead of the coreinfo when you want to use architectures other than x86. We provide the libpayload which is a small BSD-licensed static library.
The details of the error is:
W: Written area will abut bottom of target region: any unused space will keep its current contents
INFO: Performing operation on 'COREBOOT' region...
ERROR: Ramstage region _postram_cbfs_cache overlapped by: fallback/payload
Makefile.inc:1171: recipe for target 'check-ramstage-overlaps' failed
make: *** [check-ramstage-overlaps] Error 1
“ERROR: undefined reference to ‘_ttb'” and “ERROR: undefined reference to ‘_ettb'”
This errors might happen when you build coreboot.rom by `make` at root directory. In this case, You need to add TTB() at your memleyout.ld.
TTB is a translation table base address for MMU. TTBR0 and TTBR1 (TTB registers) hold the start point of TTB. We can put TTB anywhere in memory as long as we store the address to TTBR.
According to the “ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition”, the difference between TTBR0 and TTBR1 is:
(B3-1345) When a PL1&0 stage 1 MMU is enabled, TTBR0 is always used. If TTBR1 is also used then:
– TTBR1 is used for the top part of the input address range
– TTBR0 is used for the bottom part of the input address range
(B4-1724) TTBCR determines which of the Translation Table Base Registers, TTBR0 or TTBR1, defines the base address for a translation table walk required for the stage 1 translation of a memory access from any mode other than Hyp mode.
TTBR0 is basically used for user processes and TTBR1 is used for kernel. However, Linux kernel only uses TTBR0 to reduce the time of context switch. (I just heard that Linux kernel starts to use TTBR1 because of security reasons such as Meltdown and Spectre.)
In coreboot, mmu_init() sets TTBR registers in arch/arm/armv7/mmu.c.
Fails to build a sample program with libpayload
We provide the libpayload which is a small BSD-licensed static library for coreboot and we also have a sample program to know how to use it. However, you might fail to build a sample program when you select the ARM architecture as a target with the following errors:
/usr/bin/ld: cannot represent machine `arm'
The reason why this problem happens is Makefile in the sample directory is old dated. So I created a CL to update current architectures that coreboot supports.
“Payload not loaded” happens when the load address of a payload is wrong. The load address should be placed in the RAM place where anyone can use. You can define the load address via CONFIG_LP_BASE_ADDRESS if you use a libpayload.
Whole operations for building coreboot.rom with a sample payload for QEMU/ARM are:
1. Build a libc and cross compiler environment.
// In coreboot/payloads/libpayload/
$ make distclean // Always needs when switching a mainboard.
$ cp configs/config.emulation-qemu-arm configs/defconfig // Or you can set up it via 'make menuconfig'
$ make defconfig
$ make install
2. Build a sample payload hello.elf.
// In coreboot/payloads/libpayload/sample
$ make // Make sure that Makefile is updated by https://review.coreboot.org/c/coreboot/+/33287
3. Build coreboot.rom with a sample payload.
// In coreboot/
$ make distclean // Always needs when switching a mainboard.
$ make menuconfig // or make defconfig
Select payload “payloads/libpayload/sample/hello.elf”
Make sure to do ‘make distclean’ before switching your board target
‘make distclean’ removes build artifacts and config files. The default archtecture in coreboot is x86, so you need to do ‘make distclean’ when you want to use other architectures.
Fails to update an existing CL on Gerrit
Gerrit is a code review tool used in coreboot project. I’m familiar with GitHub and I thought the operations of Gerrit are almost the same with the operations of GitHub, but it weren’t.
On GitHub, developers can create a commit for each update. On the other hand, developers using Gerrit need to amend their commit until it will be merged.
Commands to create a new CL are almost the same with the operations of GitHub:
Hello again! This is a continuation of my posts about fixing the Coverity issues in coreboot. This week’s plan was to tackle the 28 issues in northbridge/intel, which turned out to be much easier than I expected, since I’m already done! With that out of the way, I’m going to begin working on northbridge/via and southbridge. For the curious, here is the project timeline for entire summer. (I had wanted to include this in last week’s post, but hadn’t figured out how to do tables in WordPress yet.)
May 6 to 10
May 13 to 17
May 20 to 24
May 27 to 31
commonlib, cpu, lib, mainboard
June 3 to 7
June 10 to 14
June 17 to 21
June 24 to 28
July 1 to 5
July 15 to 19
July 22 to 26
July 29 to Aug 2
Aug 5 to 9
Aug 12 to 16
Aug 19 to 23
As you can see, there are a lot of issues in the AMD vendorcode. This consists primarily of AGESA, AMD’s framework for initialization of their 64 bit platforms (somewhat similar to Intel’s FSP). This code is somewhat … dense (someone on IRC described it as a “sea of abstraction”), so I made sure to leave plenty of time for it. As always, you can keep up to date on my current progress on Gerrit.
PS: As an extra bonus, here is a picture of my new BeagleBone Black!
I recently got a ThinkPad T500 to practice installing coreboot on, and I needed some sort of external programmer to flash the SOIC. There are many options available (flashrom has a whole list here), but a single-board computer like this is one of the closest you can get to “plug-and-play.” There are many other popular boards (notably the Raspberry Pi), but the BBB doesn’t require any binary blobs to boot, and is open source hardware too. The only thing I’m waiting for now is an Atheros ath9k wireless card, which runs without any binary firmware. (Hey, if you’re gonna go freedom, you gotta go all the way.)
Hello everyone! My name is Jacob Garber, and I am a student in this year’s GSoC 2019! My project is on making coreboot Coverity clean. Coverity is a free static-analysis tool for open source projects that searches for common coding mistakes and errors, such as buffer overruns, null pointer dereferences, and integer overflow. Coverity automatically analyzes the coreboot codebase and flags issues it finds, and my job is to classify them into bugs and false-positives and patch them if I can. You can check the Coverity overview for coreboot here, though seeing the issue tracker itself requires registration. At the beginning of the summer, coreboot had over 380 flagged issues, but it’s now down to 303, so we’re making progress! I plan to address 20-30 issues per week depending on the source component, which so far has gone surprisingly well (surprising, in the sense that coming into the summer I knew very little about coreboot or firmware development in general). For the curious, you can see the history and progress of all my changes on Gerrit. My mentors for this project are Patrick Georgi, Martin Roth, and David Hendricks, who have all been extremely helpful in guiding me through the development process, reviewing my patches, and answering my many questions. Thank you all.
Now, fixing Coverity bugs isn’t the only thing I’d like to do this summer. As I said before, I’d like to learn more about coreboot, and what better way to do that than installing it on a laptop! My current laptop is an old 2011 Macbook Air, which is surprisingly close to getting coreboot support (many thanks to Evgeny Zinoviev). However, I am (slightly) hesitant about installing yet-experimental firmware on my one and only development machine, so until then I picked up an old Thinkpad T500 to practice on. This laptop has the advantage of being able to run blob-free, and if in the very unlikely event I end up bricking it, who cares! (I mean, I’ll care, but it was a worthy sacrifice.) I also bought a BeagleBone Black to try out external flashing and was hoping to include a picture today, but the shipping was delayed. You’ll have to wait until next week!
GSoC 2016 coding period has come to an end and mentor’s evaluating students this week. It has been an enriching 13 weeks of reading datasheets, designing structures, coding, learning and hanging out over IRC! 😛 I’d like to take this opportunity to present my work and details on how to use it. 🙂
Firstly, to offer context to the work, here is a list of public mails and blog posts. These should give an idea as to how the discussions and work evolved. A lot of the discussions have happened over IRC, but #flashrom does not keep any logs.
You can also find these over at flashrom’s patchwork. The mailing list is where the review happens (although a better alternative, IMHO, is Gerrit which coreboot uses). The patches aren’t currently merged and are under review. In any case, you are most welcome to join review (which will likely be very helpful for me). 🙂 If you’d like to look at something more on the bleeding edge, then I invite you to my GitHub.
Now, moving on how to use the work. The most exhaustive documentation on how to use it is the code itself :P, but in the following list I attempt to list scenarios –
For SPI chips that have multiple status registers, flashrom’s verbose output will print the status register bits and there values. Most bits are named, i.e., the datasheet refers to the bit by an abbreviation, for instance, WEL for Write Enable Latch, WIP for Work In Progress, BP for Block Protect, LB for Lock Bit and so on. The verbose output will print these names, both in abbreviated and long forms, for most chips (and these abbreviations tend to be generic across many manufacturers). However, the process for adding new chips that leverage this, and adding new bits, is a fairly easy task (I would invite you to have a look at the code 😉 for more details). The verbose output also prints the write protection mode for status register(s) in effect (software protected, hardware protected, power cycle lock down and so on).
In case you want to disable or enable (a particular type) write protection for status register, you can use the --wp-disable or --wp-enable[=MODE] respectively (where MODE is either of software, hardware, power cycle or permanent – you are encouraged to have a look at the man page 🙂 for more details)
In case you want to protect a particular range of an SPI chip from writes or erases, you will need to alter the BP, TB or SEC bits. Currently, there is a CLI that will enable you to accomplish all that. 😛 First, you’ll want to look at the list of ranges your SPI chip supports – run flashrom with --wp-list. Take note of the start address and the length of the memory range you want to protect. Then again run flashrom with --wp-set-range start=0xfff000,len=4 (0xfff000 and 4 are for representational purpose only). By now the memory range is protected, but you can additionally enable status register write protection by following what the foregoing point described.
For SPI chips that support OTP, you can read, write and erase OTP regions (of course for supported chips :P). For OTP operations, you have at your disposal --print-otp-status, --read-otp [,reg=], --write-otp file=[,reg=], --erase-otp [reg=] and --lock-otp [reg=]. You can read the OTP memory to a file, or you can write to the OTP region from a file, very much like reading and writing from/to SPI chip. For more details, I would again like to point you to the man page. 🙂
Since this is a work-in-progress, the CLI may change (and is very likely). Currently around 10% of SPI chips use this new infrastructure. Models of a few manufacturers (and especially exotic ones like Atmel) are yet to be fully incorporated. You are most welcome to add support for new chips or update the existing ones to support new infrastructure. 🙂
I would like to sincerely thank my mentors Stefan and David for their support and help. I am indebted to them for this opportunity and I hope that we continue to share this relationship in the future while I continue to explore and contribute to flashrom. It has been a pleasure getting to know each of them. I’d also like to thank Urja for pitching in from time to time 🙂 It was fun hanging out over IRC and helping folks asking questions there. And I am looking forward to it for years to come. 😛
In the next and final part of this post, I will highlight how we intend to improve upon this work in the future, where it will be headed and what more we have in store, so please stay tuned. 😉 Phew, this was a long one, and rightly so as it attempts to summarise a great deal of experiences. If you have any feedback, questions or comments on the blogs or code, please feel to ping me on #flashrom where I am known as hatim. You can also email me at firstname.lastname@example.org.
Thanks, and looking forward to hearing from you. 🙂 See you in the next and final part.
Compiling spike, the RISC-V instruction-set-level simulator
Spike, also known as riscv-isa-sim, is the reference implementation of RISC-V, and the only RISC-V platform that is currently known to work with coreboot (QEMU is nominally also supported, but the corresponding coreboot code has not been updated in a while).
Apply this patch that allows linux to be started in machine-mode.
Download a copy linux 4.6.x from kernel.org, and unpack it. I’ll assume version 4.6.2 is used.
cd into linux-4.6.2/arch and symlink the arch/riscv directory from riscv-linux
Back in linux-4.6.2, run make O=build ARCH=riscv defconfig; cd into the newly created build directory.
Run make ARCH=riscv menuconfig. In the “General Setup” menu of the linux menuconfig, enter path/to/coreboot/util/crossgcc/xgcc/bin/riscv64-elf- as the cross-compiler tool prefix.
Run make ARCH=riscv vmlinux to compile linux.
Open vmlinux in a hex editor, such as hexer. Change the 8-byte number at 0x18 to 00 00 00 90 00 00 00 00; Add 00 00 00 90 00 00 00 00 to the numbers at 0x58 and 0x90, to load linux at physical addresses within RAM. It’s unfortunate that I have to recommend this step, but I did not come up with a better fix yet.
Next, you need to add vmlinux to coreboot:
Return to the coreboot directory.
Apply the remaining coreboot patches that are tagged riscv.
In the Payload menu, select An ELF executable payload. Instead of payload.elf, select the vmlinux file.
Run make and util/riscvtools/make-spike-elf.sh build/coreboot.rom build/coreboot.elf again.
Run spike build/coreboot.elf. You should now see a Linux boot, at least partially.
Even though my GSoC is over, coreboot’s support for RISC-V can still be improved, and I intend to fix at least some of the following things:
As you can see above, running coreboot and linux on RISC-V currently isn’t straight-forward, but involves a few manual steps.
There are other RISC-V platforms that I’d like to see coreboot on, such as lowRISC.
Linux does not completely boot, i.e. into userspace. There are still some bugs to be ironed out.
Automatic testing could be used to detect regressions.
I only tested coreboot on RISC-V with Linux; support for other operating systems or payloads is welcome.
I’d like to thank Ron Minnich and Furquan Shaikh for being good mentors, and everyone in the coreboot community for being helpful and friendly.
This past week, I worked on the virtual memory initialization code of coreboot on RISC-V. The first part of this was to update encoding.h a file that defines constants such as bit masks which are necessary for interacting with RISC-V’s Control and Status Registers. As a result, I also had to change a few files that relied on outdated constants. Then I wrote some code to walk the page table structures, and fixed one or two bugs in the page table setup code. Unfortunately my patches aren’t as finished as I would like them to be.
When I tested my changes with a Linux payload (which I had to edit the ELF headers of, because Linux uses virtual addresses, while cbfstool and the SELF loader use physical addresses), I stumbled upon another strange error: I get a store access fault in the instruction of the trap handler that saves the first register, even if I initialize the stack pointer to a value that’s within the RAM. When the trap handler runs again to handle this store access fault, it runs without any problems. This fault is especially confusing, because machine mode should always be able to access RAM through its physical addresses.
During the next week, I’ll be traveling and won’t be able to work on coreboot. When I return, I will rework my patches so they can be merged, and hopefully understand the aforementioned access fault problem. Properly set-up page tables should bring me a step closer to running Linux on coreboot/RISC-V (without bbl in the middle).
If time permits, I will start porting coreboot to the Nexys 4 DDR devboard.