Re: [RFC PATCH 01/19] OvmfPkg: Reserve the Secrets and Cpuid page for the SEV-SNP guest

["Laszlo Ersek" <lersek@redhat.com>]

On 04/07/21 02:21, Xu, Min M wrote:

> Intel TDX also has metadata which is consumed by QEMU. We put the metadata
> in a single file (TdxMetadata.asm) and put it at the end of ResetVectorVtf0.
> Then a pointer is placed in a known location in ResetVector.nasm. In this way
> QEMU can easily read the Metadata by the pointer.
> ------------------------------------------------------------------
> ALIGN   8
> ;
> ; TDX Virtual Firmware injects metadata in VTF0.
> ; The address of the metadata is injected in this location (0xffffffe8)
> ;
>     DD      (OVMF_IMAGE_SIZE_IN_KB * 1024 - (fourGigabytes - TdxMetadataGuid - 16))
> ;
> ; The VTF signature
> ;
> ; VTF-0 means that the VTF (Volume Top File) code does not require
> ; any fixups.
> ;
> vtfSignature:
>     DB      'V', 'T', 'F', 0
> ------------------------------------------------------------------
> 
> The space in ResetVector is very precious and we all want a known location so that QEMU
> can find the metadata easily. Putting the metadata in a single file give the developers
> more flexible (They can put anything they want). So I think a pointer (point to a metadata
> file) in a known location maybe a better solution.

Assuming a QEMU version has been released that looks for the chain of
GUID-ed structs already, then I think such a change would break
compatibility with that QEMU version.

If we definitely need a separate spot to include more information in the
flash, for QEMU's parsing, then please introduce a new GUIDed structure,
which contains nothing but a pointer to that spot.

Thanks
Laszlo