Re: [PATCH] OvmfPkg/MemEncryptSevLib: check CPUID when read msr during PEI phase

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

On 11/25/21 7:12 AM, qi zhou wrote:
>  From 5b10265fa5c7b5ca728b4f18488089de6535ed28 Mon Sep 17 00:00:00 2001
> From: Qi Zhou <atmgnd@outlook.com>
> Date: Thu, 25 Nov 2021 20:25:55 +0800
> Subject: [PATCH] OvmfPkg/MemEncryptSevLib: check CPUID when read msr during
>   PEI phase
> 
> Tested on Intel Platform, It is like 'SEV-ES work area' can be modified by
> os(Windows etc), and will not restored on reboot, the
> SevEsWorkArea->EncryptionMask may have a random value after reboot. then it
> may casue fail on reboot. The msr bits already cached by mSevStatusChecked,
> there is no need to try cache again in PEI phase.
> 
> Signed-off-by: Qi Zhou <atmgnd@outlook.com>
> ---
>   .../PeiMemEncryptSevLibInternal.c             | 55 +++++++------------
>   1 file changed, 19 insertions(+), 36 deletions(-)
> 
> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLibInternal.c b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLibInternal.c
> index e2fd109d12..0819f50669 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLibInternal.c
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLibInternal.c
> @@ -38,49 +38,32 @@ InternalMemEncryptSevStatus (
>     UINT32                            RegEax;
>     MSR_SEV_STATUS_REGISTER           Msr;
>     CPUID_MEMORY_ENCRYPTION_INFO_EAX  Eax;
> -  BOOLEAN                           ReadSevMsr;
> -  SEC_SEV_ES_WORK_AREA              *SevEsWorkArea;
>   
> -  ReadSevMsr = FALSE;
> -
> -  SevEsWorkArea = (SEC_SEV_ES_WORK_AREA *) FixedPcdGet32 (PcdSevEsWorkAreaBase);
> -  if (SevEsWorkArea != NULL && SevEsWorkArea->EncryptionMask != 0) {
> -    //
> -    // The MSR has been read before, so it is safe to read it again and avoid
> -    // having to validate the CPUID information.
> +  //
> +  // Check if memory encryption leaf exist
> +  //
> +  AsmCpuid (CPUID_EXTENDED_FUNCTION, &RegEax, NULL, NULL, NULL);
> +  if (RegEax >= CPUID_MEMORY_ENCRYPTION_INFO) {

This now defeats the purpose of the workarea the already validated CPUID 
information. This CPUID information will now require validating.

Wouldn't the best thing be to clear the workarea in the early boot code?

Thanks,
Tom

>       //
> -    ReadSevMsr = TRUE;
> -  } else {
> +    // CPUID Fn8000_001F[EAX] Bit 1 (Sev supported)
>       //
> -    // Check if memory encryption leaf exist
> -    //
> -    AsmCpuid (CPUID_EXTENDED_FUNCTION, &RegEax, NULL, NULL, NULL);
> -    if (RegEax >= CPUID_MEMORY_ENCRYPTION_INFO) {
> +    AsmCpuid (CPUID_MEMORY_ENCRYPTION_INFO, &Eax.Uint32, NULL, NULL, NULL);
> +
> +    if (Eax.Bits.SevBit) {
>         //
> -      // CPUID Fn8000_001F[EAX] Bit 1 (Sev supported)
> +      // Check MSR_0xC0010131 Bit 0 (Sev Enabled)
>         //
> -      AsmCpuid (CPUID_MEMORY_ENCRYPTION_INFO, &Eax.Uint32, NULL, NULL, NULL);
> -
> -      if (Eax.Bits.SevBit) {
> -        ReadSevMsr = TRUE;
> +      Msr.Uint32 = AsmReadMsr32 (MSR_SEV_STATUS);
> +      if (Msr.Bits.SevBit) {
> +        mSevStatus = TRUE;
>         }
> -    }
> -  }
> -
> -  if (ReadSevMsr) {
> -    //
> -    // Check MSR_0xC0010131 Bit 0 (Sev Enabled)
> -    //
> -    Msr.Uint32 = AsmReadMsr32 (MSR_SEV_STATUS);
> -    if (Msr.Bits.SevBit) {
> -      mSevStatus = TRUE;
> -    }
>   
> -    //
> -    // Check MSR_0xC0010131 Bit 1 (Sev-Es Enabled)
> -    //
> -    if (Msr.Bits.SevEsBit) {
> -      mSevEsStatus = TRUE;
> +      //
> +      // Check MSR_0xC0010131 Bit 1 (Sev-Es Enabled)
> +      //
> +      if (Msr.Bits.SevEsBit) {
> +        mSevEsStatus = TRUE;
> +      }
>       }
>     }
>   
>