Re: [edk2-devel] [PATCH 09/10] OvmfPkg/ResetVector: leave SEV VC handler installed longer

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

On 2/22/24 05:54, Gerd Hoffmann wrote:
> When running in SEV mode keep the VC handler installed.
> Add a function to uninstall it later.
> 
> 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>

Looks good, just one minor comment below.

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

> ---
>   OvmfPkg/ResetVector/Ia32/AmdSev.asm       | 12 ++++++++++--
>   OvmfPkg/ResetVector/Ia32/PageTables64.asm |  1 +
>   OvmfPkg/ResetVector/Main.asm              |  4 ++++
>   3 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/OvmfPkg/ResetVector/Ia32/AmdSev.asm b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> index ed94f1dc668f..9063ce1080d3 100644
> --- a/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> +++ b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> @@ -320,9 +320,9 @@ NoSevEsVcHlt:
>   NoSevPass:
>       xor       eax, eax
>   
> -SevExit:
>       ;
> -    ; Clear exception handlers and stack
> +    ; When NOT running in SEV mode: clear exception handlers and stack here.
> +    ; Otherwise: SevClearVcHandlerAndStack must be called later.
>       ;
>       push      eax
>       mov       eax, ADDR_OF(IdtrClear)
> @@ -330,8 +330,16 @@ SevExit:
>       pop       eax
>       mov       esp, 0
>   
> +SevExit:
>       OneTimeCallRet CheckSevFeatures
>   
> +SevClearVcHandlerAndStack:
> +    ; Clear exception handlers and stack
> +    mov       eax, ADDR_OF(IdtrClear)
> +    lidt      [cs:eax]
> +    mov       esp, 0
> +    OneTimeCallRet SevClearVcHandlerAndStack
> +
>   ; Start of #VC exception handling routines
>   ;
>   
> diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> index ada3dc0ffbe0..6e2063430802 100644
> --- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> +++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> @@ -250,6 +250,7 @@ SevInit:
>       CreatePageTables4Level edx
>       ; Clear the C-bit from the GHCB page if the SEV-ES is enabled.
>       OneTimeCall   SevClearPageEncMaskForGhcbPage
> +    OneTimeCall   SevClearVcHandlerAndStack
>       jmp SetCr3
>   
>   TdxBspInit:
> diff --git a/OvmfPkg/ResetVector/Main.asm b/OvmfPkg/ResetVector/Main.asm
> index 46cfa87c4c0a..88b25db3bc9e 100644
> --- a/OvmfPkg/ResetVector/Main.asm
> +++ b/OvmfPkg/ResetVector/Main.asm
> @@ -80,7 +80,11 @@ SearchBfv:
>       ; Set the OVMF/SEV work area as appropriate.
>       ;
>       OneTimeCall CheckSevFeatures
> +    cmp         byte[WORK_AREA_GUEST_TYPE], 1
> +    jnz         NoSevIa32
> +    OneTimeCall SevClearVcHandlerAndStack

I think it is safe to invoke SevClearVcHandlerAndStack no matter what if 
you want to avoid the cmp and jnz instructions.

Thanks,
Tom

>   
> +NoSevIa32:
>       ;
>       ; Restore initial EAX value into the EAX register
>       ;


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