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Message-ID: <1a3502d6-98e3-463c-8923-28217b5e64a0@amd.com>
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;
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