linux-kernel - Re: [PATCH 07/11] x86/sev: Provide SVSM discovery support

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Date: Mon, 29 Jan 2024 09:18:01 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Dan Williams <dan.j.williams@...el.com>, Michael Roth
 <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH 07/11] x86/sev: Provide SVSM discovery support

On 1/29/24 04:41, Jeremi Piotrowski wrote:
> On Fri, Jan 26, 2024 at 04:16:00PM -0600, Tom Lendacky wrote:
>> The SVSM specification documents an alternative method of discovery for
>> the SVSM using a reserved CPUID bit and a reserved MSR.
>>
>> For the CPUID support, the #VC handler of an SEV-SNP guest should modify
>> the returned value in the EAX register for the 0x8000001f CPUID function
>> by setting bit 28 when an SVSM is present.
>>
> 
> It seems awkward that the guest would have to set the bit in the CPUID response
> itself. Is there no way for the SVSM to fixup the CPUID page contents, or the
> CPUID instruction return?

It's possible for the SVSM to update the CPUID information provided to the 
guest, but it hasn't been specifically called out as a requirement in the 
SVSM spec. That is why snp_cpuid_postprocess() is used to ensure that the 
bit is set. But, yes, we could update the CPUID information in the CPUID 
page directly instead of the snp_cpuid_postprocess() change.

I'll look into that and see what it takes.

> 
> Could you also add a definition for the feature to arch/x86/include/asm/cpufeatures.h.

Yes, I missed that, will add it.

Thanks,
Tom

> 
>> For the MSR support, new reserved MSR 0xc001f000 has been defined. A #VC
>> should be generated when accessing this MSR. The #VC handler is expected
>> to ignore writes to this MSR and return the physical calling area address
>> (CAA) on reads of this MSR.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>> ---
>>   arch/x86/include/asm/msr-index.h |  2 ++
>>   arch/x86/kernel/sev-shared.c     |  4 ++++
>>   arch/x86/kernel/sev.c            | 17 +++++++++++++++++
>>   3 files changed, 23 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
>> index f1bd7b91b3c6..4746135cbe21 100644
>> --- a/arch/x86/include/asm/msr-index.h
>> +++ b/arch/x86/include/asm/msr-index.h
>> @@ -622,6 +622,8 @@
>>   
>>   #define MSR_AMD64_VIRT_SPEC_CTRL	0xc001011f
>>   
>> +#define MSR_SVSM_CAA			0xc001f000
>> +
>>   /* AMD Collaborative Processor Performance Control MSRs */
>>   #define MSR_AMD_CPPC_CAP1		0xc00102b0
>>   #define MSR_AMD_CPPC_ENABLE		0xc00102b1
>> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
>> index f26e872bc5d0..9bd7d7e75b31 100644
>> --- a/arch/x86/kernel/sev-shared.c
>> +++ b/arch/x86/kernel/sev-shared.c
>> @@ -628,6 +628,10 @@ static int snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
>>   		/* node ID */
>>   		leaf->ecx = (leaf->ecx & GENMASK(31, 8)) | (leaf_hv.ecx & GENMASK(7, 0));
>>   		break;
>> +	case 0x8000001f:
>> +		if (vmpl)
>> +			leaf->eax |= BIT(28);
>> +		break;
>>   	default:
>>   		/* No fix-ups needed, use values as-is. */
>>   		break;