Re: [PATCH 4/4] OvmfPkg/CcExitLib: Use documented XSave area base size for SEV-SNP

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 12/21/22 10:06, Michael Roth wrote:
> Currently OVMF tries to rely on the base size advertised via the CPUID
> table entries corresponding to leaf 0xD, sub-leafs 0x0/0x1. This will
> generally work for KVM guests, but might not for other SEV-SNP
> hypervisor implementations. Make the handling more robust by simply
> using the base area size documented by the APM.
> 
> Signed-off-by: Michael Roth <michael.roth@amd.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>   OvmfPkg/Library/CcExitLib/CcExitVcHandler.c | 9 ++++-----
>   1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> index cd117d5a31..f985dcff8d 100644
> --- a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> +++ b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> @@ -1647,8 +1647,6 @@ SnpEnabled (
>   
>     @param[in]      XFeaturesEnabled  Bit-mask of enabled XSAVE features/areas as
>                                       indicated by XCR0/MSR_IA32_XSS bits
> -  @param[in]      XSaveBaseSize     Base/legacy XSAVE area size (e.g. when
> -                                    XCR0 is 1)
>     @param[in, out] XSaveSize         Pointer to storage for calculated XSAVE area
>                                       size
>     @param[in]      Compacted         Whether or not the calculation is for the
> @@ -1663,7 +1661,6 @@ STATIC
>   BOOLEAN
>   GetCpuidXSaveSize (
>     IN     UINT64   XFeaturesEnabled,
> -  IN     UINT32   XSaveBaseSize,
>     IN OUT UINT32   *XSaveSize,
>     IN     BOOLEAN  Compacted
>     )
> @@ -1672,7 +1669,10 @@ GetCpuidXSaveSize (
>     UINT64              XFeaturesFound = 0;
>     UINT32              Idx;
>   
> -  *XSaveSize = XSaveBaseSize;
> +  //
> +  // The base/legacy XSave size is documented to be 0x240 in the APM.
> +  //
> +  *XSaveSize = 0x240;
>     CpuidInfo  = (SEV_SNP_CPUID_INFO *)(UINT64)PcdGet32 (PcdOvmfCpuidBase);
>   
>     for (Idx = 0; Idx < CpuidInfo->Count; Idx++) {
> @@ -1888,7 +1888,6 @@ GetCpuidFw (
>   
>       if (!GetCpuidXSaveSize (
>              XCr0 | XssMsr.Uint64,
> -           *Ebx,
>              &XSaveSize,
>              Compacted
>              ))