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Subject: Re: [PATCH 6/9] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Set the SwScratch valid bit
To: Tom Lendacky <thomas.lendacky@amd.com>, devel@edk2.groups.io
Cc: Brijesh Singh <brijesh.singh@amd.com>,
 Jordan Justen <jordan.l.justen@intel.com>,
 Ard Biesheuvel <ard.biesheuvel@arm.com>
References: <cover.1602346027.git.thomas.lendacky@amd.com>
 <f383fb32d6d3bcaa7a5cba7595d7206f45c53fb4.1602346027.git.thomas.lendacky@amd.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <260a19c6-e1f4-14e9-2ecd-8188d2fe596a@redhat.com>
Date: Thu, 15 Oct 2020 11:25:40 +0200
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On 10/10/20 18:07, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> All fields that are set in the GHCB should have their associated bit in
> the GHCB ValidBitmap field set. Add support to set the bit for the scratch
> area field (SwScratch).
> 
> Fixes: 437eb3f7a8db ("OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Bypass flash detection with SEV-ES")
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> index 565383ee26d2..5d5a117c48e0 100644
> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
> @@ -52,10 +52,15 @@ QemuFlashPtrWrite (
>    if (MemEncryptSevEsIsEnabled ()) {
>      MSR_SEV_ES_GHCB_REGISTER  Msr;
>      GHCB                      *Ghcb;
> +    UINT32                    ScratchIndex;
> +    UINT32                    ScratchBit;
>  
>      Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
>      Ghcb = Msr.Ghcb;
>  
> +    ScratchIndex = GhcbSwScratch / 8;
> +    ScratchBit   = GhcbSwScratch & 0x07;
> +
>      //
>      // Writing to flash is emulated by the hypervisor through the use of write
>      // protection. This won't work for an SEV-ES guest because the write won't
> @@ -66,6 +71,7 @@ QemuFlashPtrWrite (
>      VmgInit (Ghcb);
>      Ghcb->SharedBuffer[0] = Value;
>      Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
> +    Ghcb->SaveArea.ValidBitmap[ScratchIndex] |= (1 << ScratchBit);
>      VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
>      VmgDone (Ghcb);
>    } else {
> 

Makes sense, but please reimplement this with the new (proposed)
VmgSetOffsetValid() library function.

Thanks
Laszlo