The Linux WARN regarding AVIC on PC Engines

[pietrushnic]

I have found out that AVIC is not supported on this HW. The CPUID 0x8000000A EDX indicates no support for AVIC. That means the IOMMU guest AVIC support should not be exposed at all. I have prepared a fix for it already by hiding the guest AVIC capability in IOMMU and disabling the feature. So far it works and no more WARNs is visible in dmesg.

Originally posted by @miczyg1 in #1134 (comment)

[miczyg1]

Fixed it here: Dasharo/coreboot#592

[pietrushnic]

@miczyg1 huge kudos for that ❤️

Do you have ideas on how I would write a test against that? If not for the upcoming release, then for the next one, I would like to have it to avoid regression. I can do this for Dasharo (coreboot+SeaBIOS), but I'm not sure if it would be useful for Dasharo (coreboot+UEFI). Is just checking for the warning in dmesg good enough? I will check if it is visible in v24.08.00.01 while working on the release.

[miczyg1]

I guess it would have to be something to check dmesg for: [ 23.736476] WARNING: CPU: 2 PID: 1 at drivers/iommu/amd/init.c:980 enable_iommus_vapic+0 . Of course ignoring the timestamp, CPU number, PID and location in the file. For example if a line contains both WARNING: CPU and enable_iommus_vapic, the test should fail.

If we want to test for this specific WARN I think there is no other way.