Re: [PATCH v6 02/29] OvmfPkg: reserve CPUID page for SEV-SNP


Brijesh Singh
 

Hi Gerd,

On 9/2/21 3:04 AM, Gerd Hoffmann wrote:
On Wed, Sep 01, 2021 at 11:16:19AM -0500, Brijesh Singh wrote:
BZ: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D3275&;data=04%7C01%7Cbrijesh.singh%40amd.com%7C13c81a39aa2e4f22430e08d96de85a69%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637661666978547521%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=4b22Sv6xoUGQ3xutPYdsqb4cNh1SS9Z8MOQG7dHiqYU%3D&reserved=0

Platform features and capabilities are traditionally discovered via the
CPUID instruction. Hypervisors typically trap and emulate the CPUID
instruction for a variety of reasons. There are some cases where incorrect
CPUID information can potentially lead to a security issue. The SEV-SNP
firmware provides a feature to filter the CPUID results through the PSP.
The filtered CPUID values are saved on a special page for the guest to
consume. Reserve a page in MEMFD that will contain the results of
filtered CPUID values.
Is the format of the page documented somewhere?
Yes, it is documented in the SEV-SNP spec [1] section 7.1 and the checks
performed by the SEV-SNP firmware are documented in the PPR [2] section
2.1.5.3. I will document these link in the commit message.

[1] https://www.amd.com/system/files/TechDocs/56860.pdf

[2]
https://www.amd.com/en/support/tech-docs/processor-programming-reference-ppr-for-amd-family-19h-model-01h-revision-b1


Is this snp-specific? Or could this also be used without snp?
This is SNP specific format and cannot be used without SNP.

thanks

Brijesh

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