Re: [PATCH V3] OvmfPkg/ResetVector: Removing SEV-ES CPUID bit check

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 31 May 2022 at 16:40, Peter Gonda <pgonda@google.com> wrote:
>
> The SEV-ES bit of Fn800-001F[EAX] - Bit 3 is used for a host to
> determine support for running SEV-ES guests. It should not be checked by
> a guest to determine if it is running under SEV-ES. The guest should use
> the SEV_STATUS MSR Bit 1 to determine if SEV-ES is enabled. This check
> was not part of the original SEV-ES support and was added in
> a91b700e38. Removing the check makes this code consistent with the
> Linux kernel
>
> Fixes: a91b700e38 (Ovmf/ResetVector: Simplify and consolidate the SEV features checks)
>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: Marc Orr <marcorr@google.com>
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
>
> ---
>  OvmfPkg/ResetVector/Ia32/AmdSev.asm | 8 --------
>  1 file changed, 8 deletions(-)
>
> diff --git a/OvmfPkg/ResetVector/Ia32/AmdSev.asm b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> index 1f827da3b9..77692db27e 100644
> --- a/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> +++ b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
> @@ -265,14 +265,6 @@ CheckSevFeatures:
>      ; Set the work area header to indicate that the SEV is enabled
>      mov     byte[WORK_AREA_GUEST_TYPE], 1
>
> -    ; Check for SEV-ES memory encryption feature:
> -    ; CPUID  Fn8000_001F[EAX] - Bit 3
> -    ;   CPUID raises a #VC exception if running as an SEV-ES guest
> -    mov       eax, 0x8000001f
> -    cpuid
> -    bt        eax, 3
> -    jnc       GetSevEncBit
> -
>      ; Check if SEV-ES is enabled
>      ;  MSR_0xC0010131 - Bit 1 (SEV-ES enabled)
>      mov       ecx, SEV_STATUS_MSR

Thanks Peter, I have queued this up.

I did wonder, though: the only remaining reference to GetSevEncBit is
a conditional branch that just precedes the label itself. This appears
to be a leftover from commit 63c50d3ff2854a76 ("OvmfPkg/ResetVector:
cache the SEV status MSR value in workarea") but it looks a bit dodgy.