Re: [edk2-devel] [PATCH 2/3] OvmfPkg/VmgExitLib: Add support for new MMIO MOV opcodes

["Laszlo Ersek" <lersek@redhat.com>]

On 04/21/21 00:54, Lendacky, Thomas wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
>
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3345
>
> Enabling TPM support results in guest termination of an SEV-ES guest
> because it uses MMIO opcodes that are not currently supported.
>
> Add support for the new MMIO opcodes (0xA0 - 0xA3), MOV instructions which
> use a memory offset directly encoded in the instruction. Also, add a DEBUG
> statement to identify an unsupported MMIO opcode being used.
>
> Fixes: c45f678a1ea2080344e125dc55b14e4b9f98483d
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c | 99 +++++++++++++++++++
>  1 file changed, 99 insertions(+)
>
> diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
> index 273f36499988..f9660b757d8e 100644
> --- a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
> +++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
> @@ -678,6 +678,7 @@ MmioExit (
>    UINTN   Bytes;
>    UINT64  *Register;
>    UINT8   OpCode, SignByte;
> +  UINTN   Address;
>
>    Bytes = 0;
>
> @@ -727,6 +728,51 @@ MmioExit (
>      }
>      break;
>
> +  //
> +  // MMIO write (MOV moffsetX, aX)
> +  //
> +  case 0xA2:
> +    Bytes = 1;
> +    //
> +    // fall through
> +    //
> +  case 0xA3:
> +    Bytes = ((Bytes != 0) ? Bytes :
> +             (InstructionData->DataSize == Size16Bits) ? 2 :
> +             (InstructionData->DataSize == Size32Bits) ? 4 :
> +             (InstructionData->DataSize == Size64Bits) ? 8 :
> +             0);
> +
> +    InstructionData->ImmediateSize = (UINTN) (1 << InstructionData->AddrSize);
> +    InstructionData->End += (UINTN) (1 << InstructionData->AddrSize);
> +
> +    if (InstructionData->AddrSize == Size8Bits) {
> +      Address = *(UINT8 *) InstructionData->Immediate;
> +    } else if (InstructionData->AddrSize == Size16Bits) {
> +      Address = *(UINT16 *) InstructionData->Immediate;
> +    } else if (InstructionData->AddrSize == Size32Bits) {
> +      Address = *(UINT32 *) InstructionData->Immediate;
> +    } else {
> +      Address = *(UINTN *) InstructionData->Immediate;
> +    }

(1) Can we simplify this as follows?

    InstructionData->ImmediateSize = 1 << InstructionData->AddrSize;
    InstructionData->End += InstructionData->ImmediateSize;
    Address = 0;
    CopyMem (&Address, InstructionData->Immediate,
      InstructionData->ImmediateSize);

> +
> +    Status = ValidateMmioMemory (Ghcb, Address, Bytes);
> +    if (Status != 0) {
> +      return Status;
> +    }
> +
> +    ExitInfo1 = Address;
> +    ExitInfo2 = Bytes;
> +    CopyMem (Ghcb->SharedBuffer, &Regs->Rax, Bytes);
> +
> +    Ghcb->SaveArea.SwScratch = (UINT64) Ghcb->SharedBuffer;
> +    VmgSetOffsetValid (Ghcb, GhcbSwScratch);
> +    Status = VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, ExitInfo1, ExitInfo2);
> +    if (Status != 0) {
> +      return Status;
> +    }
> +    break;
> +
>    //
>    // MMIO write (MOV reg/memX, immX)
>    //
> @@ -809,6 +855,58 @@ MmioExit (
>      CopyMem (Register, Ghcb->SharedBuffer, Bytes);
>      break;
>
> +  //
> +  // MMIO read (MOV aX, moffsetX)
> +  //
> +  case 0xA0:
> +    Bytes = 1;
> +    //
> +    // fall through
> +    //
> +  case 0xA1:
> +    Bytes = ((Bytes != 0) ? Bytes :
> +             (InstructionData->DataSize == Size16Bits) ? 2 :
> +             (InstructionData->DataSize == Size32Bits) ? 4 :
> +             (InstructionData->DataSize == Size64Bits) ? 8 :
> +             0);
> +
> +    InstructionData->ImmediateSize = (UINTN) (1 << InstructionData->AddrSize);
> +    InstructionData->End += (UINTN) (1 << InstructionData->AddrSize);
> +
> +    if (InstructionData->AddrSize == Size8Bits) {
> +      Address = *(UINT8 *) InstructionData->Immediate;
> +    } else if (InstructionData->AddrSize == Size16Bits) {
> +      Address = *(UINT16 *) InstructionData->Immediate;
> +    } else if (InstructionData->AddrSize == Size32Bits) {
> +      Address = *(UINT32 *) InstructionData->Immediate;
> +    } else {
> +      Address = *(UINTN *) InstructionData->Immediate;
> +    }

(2) Similar question as (1).

> +
> +    Status = ValidateMmioMemory (Ghcb, Address, Bytes);
> +    if (Status != 0) {
> +      return Status;
> +    }
> +
> +    ExitInfo1 = Address;
> +    ExitInfo2 = Bytes;
> +
> +    Ghcb->SaveArea.SwScratch = (UINT64) Ghcb->SharedBuffer;
> +    VmgSetOffsetValid (Ghcb, GhcbSwScratch);
> +    Status = VmgExit (Ghcb, SVM_EXIT_MMIO_READ, ExitInfo1, ExitInfo2);
> +    if (Status != 0) {
> +      return Status;
> +    }
> +
> +    if (Bytes == 4) {
> +      //
> +      // Zero-extend for 32-bit operation
> +      //
> +      Regs->Rax = 0;
> +    }

(3) This is also seen with opcode 0x8B, but can you remind me please why
we ignore (Bytes == 1) and (Bytes == 2) for zero extension?

> +    CopyMem (&Regs->Rax, Ghcb->SharedBuffer, Bytes);
> +    break;
> +
>    //
>    // MMIO read w/ zero-extension ((MOVZX regX, reg/memX)
>    //
> @@ -886,6 +984,7 @@ MmioExit (
>      break;
>
>    default:
> +    DEBUG ((DEBUG_INFO, "Invalid MMIO opcode (%x)\n", OpCode));
>      Status = GP_EXCEPTION;
>      ASSERT (FALSE);
>    }
>

(4) We should use the DEBUG_ERROR log mask here.

Thanks
Laszlo