Re: [PATCH v3 2/9] KVM: selftests: Add a basic SNP smoke test

[Pratik R. Sampat]

Hi Sean,

On 10/30/2024 12:57 PM, Sean Christopherson wrote:
> On Wed, Oct 30, 2024, Pratik R. Sampat wrote:
>> On 10/30/2024 8:46 AM, Sean Christopherson wrote:
>>> +/* Minimum firmware version required for the SEV-SNP support */
>>> +#define SNP_FW_REQ_VER_MAJOR   1
>>> +#define SNP_FW_REQ_VER_MINOR   51
>>>
>>> Side topic, why are these hardcoded?  And where did they come from?  If they're
>>> arbitrary KVM selftests values, make that super duper clear.
>>
>> Well, it's not entirely arbitrary. This was the version that SNP GA'd
>> with first so that kind of became the minimum required version needed.
>>
>> I think the only place we've documented this is here -
>> https://github.com/AMDESE/AMDSEV/tree/snp-latest?tab=readme-ov-file#upgrade-sev-firmware.
>>
>> Maybe, I can modify the comment above to say something like -
>> Minimum general availability release firmware required for SEV-SNP support.
> 
> Hmm, so if AMD says SNP is only supported for firmware version >= 1.51, why on
> earth is that not checked and enforced by the kernel?  Relying on userspace to
> not crash the host (or worse) because of unsupported firmware is not a winning
> strategy.

We do check against the firmware level 1.51 while setting things up
first (drivers/crypto/ccp/sev-dev.c:__sev_snp_init_locked()) and we bail
out if it's otherwise. From the userspace, calls to KVM_SEV_INIT2 or any
other corresponding SNP calls should fail cleanly without any adverse
effects to the host.

>From the positive selftest perspective though, we want to make sure it's
both supported and enabled, and skip the test if not.
I believe we can tell if it's supported by the platform using the MSR -
MSR_AMD64_SEV_SNP_ENABLED or the X86_FEATURE_SEV_SNP from the KVM
capabilities. However, to determine if it's enabled from the kernel, I
made this check here. Having said that, I do agree that there should
probably be a better way to expose this support to the userspace.

Thanks
Pratik