Re: [edk2-devel] [PATCH 15/16] Ovmfpkg/CcExitLib: Provide SVSM discovery support

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

On 1/29/24 09:23, Gerd Hoffmann via groups.io wrote:
> On Fri, Jan 26, 2024 at 04:13:14PM -0600, Tom Lendacky wrote:
>> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654
>>
>> 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.
>>
>> 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.
> 
> I'm wondering why this is handled by the ovmf #vc handler instead of the
> svsm #vc handler?

OVMF will be running in guest VMPL1 context, not SVSM VMPL0 context, when 
the CPUID instruction / MSR access is performed. In that case, it is the 
guest VMPL1 #VC handler that will get control and have to respond.

Thanks,
Tom

> 
> take care,
>    Gerd
> 
> 
> 
> 
> 
> 


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