Re: [OE-core] [wrynose][PATCH] qemuriscv.inc: use fw_dynamic.elf as BIOS when KERNEL_DEBUG is enabled

["Trevor Gamblin" <tgamblin@baylibre.com>]

On Thu Sep 17, 2026 at 3:06 AM EDT, Yoann Congal wrote:
> On Wed Sep 16, 2026 at 9:55 PM CEST, Trevor Gamblin via lists.openembedded.org wrote:
>> On Wed Sep 16, 2026 at 8:15 AM EDT, Harish.Sadineni wrote:
>>> From: Harish Sadineni <Harish.Sadineni@windriver.com>
>>>
>>> QEMU RISC-V hangs (qemu hang-up) when using fw_jump.elf as the
>>> default BIOS while kernel debugging is enabled. This has been
>>> observed with the 6.18 kernel (exact version where it was
>>> introduced is not known).
>>>
>>> As a workaround, switch to fw_dynamic.elf when KERNEL_DEBUG is
>>> enabled, which avoids the hang and works correctly with debug
>>> builds. The default behaviour is unchanged and continues to use
>>> fw_jump.elf when KERNEL_DEBUG is not set.
>>
>> It'd be good to understand exactly why this doesn't work and what changed
>> upstream. I'm looking into it in a local checkout, but I probably won't have an
>> answer for a day or two.
>
> Thanks Trevor!
>
> Harish, this patch is for master isn't it?
> If that's the case, would you mind resending it witout the [wrynose] tag
> so it can be picked-up there?

One of my colleagues recently ran into a similar issue working on meta-cheri,
and remedied it in the same way:

https://github.com/CHERI-Alliance/meta-cheri/commit/c81a323888357acc1ee244fbf8f23df4b1c1288b

I went and compared kernel image sizes across four configurations:

|6.18:
|
|lrwxrwxrwx. 2 tgamblin tgamblin  1M Sep 17 13:51 Image -> Image--6.18.52+git0+9e9d8b1f91_08edce0ecd-r0-qemuriscv64-20260917173344.bin
|-rw-r--r--. 2 tgamblin tgamblin 27M Sep 17 13:51 Image--6.18.52+git0+9e9d8b1f91_08edce0ecd-r0-qemuriscv64-20260917173344.bin
|
|6.18, KERNEL_DEBUG = "True":
|
|lrwxrwxrwx. 2 tgamblin tgamblin  1M Sep 17 14:08 Image -> Image--6.18.52+git0+9e9d8b1f91_08edce0ecd-r0-qemuriscv64-20260917180541.bin
|-rw-r--r--. 2 tgamblin tgamblin 33M Sep 17 14:08 Image--6.18.52+git0+9e9d8b1f91_08edce0ecd-r0-qemuriscv64-20260917180541.bin
|
|7.2:
|
|lrwxrwxrwx. 2 tgamblin tgamblin  1M Sep 17 14:33 Image -> Image--7.2.6+git0+b5d69636e3_d8a1ec518a-r0-qemuriscv64-20260917183100.bin
|-rw-r--r--. 2 tgamblin tgamblin 27M Sep 17 14:33 Image--7.2.6+git0+b5d69636e3_d8a1ec518a-r0-qemuriscv64-20260917183100.bin
|
|7.2, KERNEL_DEBUG = "True":
|
|lrwxrwxrwx. 2 tgamblin tgamblin  1M Sep 17 14:15 Image -> Image--7.2.6+git0+b5d69636e3_d8a1ec518a-r0-qemuriscv64-20260917181151.bin
|-rw-r--r--. 2 tgamblin tgamblin 33M Sep 17 14:15 Image--7.2.6+git0+b5d69636e3_d8a1ec518a-r0-qemuriscv64-20260917181151.bin

It's no surprise that turning on debug increases the image size, but it's a
little weird that 7.2 is fine when 6.18 isn't. My guess is that the content
where the FDT would overlap is different. I took the time to try a memory
dump from the QEMU monitor in the hanging case, and I could see the FDT magic
string where some kernel data should be:

1. Do 'runqemu nographic snapshot'
2. CTRL+A then 'c'
3. Do 'pmemsave 0x82200000 0x2000 dtb_dump.bin'
4. Inspect it like:

|tgamblin@megalith ~/workspace/ypbuilds/poky-qemuriscv64/build $ od -An -tx1 -N16 dtb_dump.bin
|d0 0d fe ed 00 00 35 ec 00 00 00 38 00 00 23 e8

'd0 0d fe ed' is in the kernel's scripts/dtc/libfdt/fdt.h:

|#define FDT_MAGIC       0xd00dfeed      /* 4: version, 4: total size */

The debug case (or other kernel customizations) may increase the kernel image
size to the point where we risk this overlap. I could dig a bit further, but I
think it's safer to use fw_dynamic.elf in general, starting with master. There
needs to be more info explaining why we did it though, both in the commit
message and as a comment near the change.
>
> Thanks!
>
>>
>> Trevor
>>>
>>> [YOCTO #16409]
> FYI, I have not seen this specific error on wrynose (kernel 6.18).
>
>>>
>>> Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
>>> ---
>>>  meta/conf/machine/include/riscv/qemuriscv.inc | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> Regards,