Re: [edk2-devel] [PATCH v3 4/6] OvmfPkg/ResetVector: SEV: keep #vc handler installed longer

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

On 2/20/24 03:06, Gerd Hoffmann wrote:
> When running in SEV mode do not uninstall the #vc handler in
> CheckSevFeatures.   Keep it active and uninstall it later in
> SevClearPageEncMaskForGhcbPage.
> 
> This allows using the cpuid instruction in SetCr3ForPageTables64,
> which is needed to check for la57 & 1G page support.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

I think a comment should be added above where the #VC handler is 
established to document that the #VC handler is removed at the end of this 
function if SEV is not active or that it remains installed to support 
CPUID calls, e.g. to check for 5-level paging support, and is removed 
later in SevClearPageEncMaskForGhcbPage().

With that,

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

> ---
>   OvmfPkg/ResetVector/Ia32/AmdSev.asm | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/OvmfPkg/ResetVector/Ia32/AmdSev.asm b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> index 043c88a7abbe..02f287f1d934 100644
> --- a/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> +++ b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> @@ -158,6 +158,11 @@ SevClearPageEncMaskForGhcbPage:
>       cmp       byte[WORK_AREA_GUEST_TYPE], 1
>       jnz       SevClearPageEncMaskForGhcbPageExit
>   
> +    ; Clear exception handlers and stack
> +    mov       eax, ADDR_OF(IdtrClear)
> +    lidt      [cs:eax]
> +    mov       esp, 0
> +
>       ; Check if SEV-ES is enabled
>       mov       ecx, 1
>       bt        [SEV_ES_WORK_AREA_STATUS_MSR], ecx
> @@ -332,7 +337,6 @@ NoSevEsVcHlt:
>   NoSevPass:
>       xor       eax, eax
>   
> -SevExit:
>       ;
>       ; Clear exception handlers and stack
>       ;
> @@ -342,6 +346,7 @@ SevExit:
>       pop       eax
>       mov       esp, 0
>   
> +SevExit:
>       OneTimeCallRet CheckSevFeatures
>   
>   ; Start of #VC exception handling routines


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