Re: [PATCH] x86/CPU/AMD: Ignore invalid reset reason value

[Borislav Petkov <bp@alien8.de>]

On July 23, 2025 9:34:26 PM GMT+03:00, Yazen Ghannam <yazen.ghannam@amd.com> wrote:
>On Tue, Jul 22, 2025 at 06:56:15PM +0200, Borislav Petkov wrote:
>> On Mon, Jul 21, 2025 at 06:11:54PM +0000, Yazen Ghannam wrote:
>> > The reset reason value may be "all bits set", e.g. 0xFFFFFFFF. This is a
>> > commonly used error response from hardware. This may occur due to a real
>> > hardware issue or when running in a VM.
>> 
>> Well, which is it Libing is reporting? VM or a real hw issue?
>> 
>
>In this case, it was a VM.
>
>> If it is a VM, is that -1 the only thing a VMM returns when reading that
>> MMIO address or can it be anything?
>> 
>> If latter, you need to check X86_FEATURE_HYPERVISOR.
>> 
>> Same for a real hw issue.
>> 
>> IOW, is -1 the *only* invalid data we can read here or are we playing
>> whack-a-mole with it?
>> 
>
>I see you're point, but I don't think we can know for sure all possible
>cases. There are some reserved bits that shouldn't be set. But these
>definitions could change in the future.
>
>And it'd be a pain to try and verify combinations of bits and configs.
>Like can bit A and B be set together, or can bit C be set while running
>in a VM, or can bit D ever be set on Model Z? 
>
>The -1 (all bits set) is the only "applies to all cases" invalid data,
>since this is a common hardware error response. So we can at least check
>for this.
>
>Thanks,
>Yazen

I think you should check both: HV or -1.

HV covers the VM angle as they don't emulate this and we simply should disable this functionality when running as a guest.

-1 covers the known-bad hw value.

Thx.

-- 
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