Re: [edk2-devel] [PATCH v8 16/46] OvmfPkg/VmgExitLib: Add support for MSR_PROT NAE events

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

On 05/19/20 23:50, Lendacky, Thomas wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198
> 
> Under SEV-ES, a MSR_PROT intercept generates a #VC exception. VMGEXIT must
> be used to allow the hypervisor to handle this intercept.
> 
> Add support to construct the required GHCB values to support an MSR_PROT
> NAE event. Parse the instruction that generated the #VC exception to
> determine whether it is RDMSR or WRMSR, setting the required register
> register values in the GHCB and creating the proper SW_EXIT_INFO1 value in
> the GHCB.
> 
> 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>
> ---
>  .../Library/VmgExitLib/X64/VmgExitVcHandler.c | 63 +++++++++++++++++++
>  1 file changed, 63 insertions(+)
> 
> diff --git a/OvmfPkg/Library/VmgExitLib/X64/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/X64/VmgExitVcHandler.c
> index 2f62795edf61..1c6b472a47c4 100644
> --- a/OvmfPkg/Library/VmgExitLib/X64/VmgExitVcHandler.c
> +++ b/OvmfPkg/Library/VmgExitLib/X64/VmgExitVcHandler.c
> @@ -411,6 +411,65 @@ UnsupportedExit (
>    return Status;
>  }
>  
> +/**
> +  Handle an MSR event.
> +
> +  Use the VMGEXIT instruction to handle either a RDMSR or WRMSR event.
> +
> +  @param[in, out] Ghcb             Pointer to the Guest-Hypervisor Communication
> +                                   Block
> +  @param[in, out] Regs             x64 processor context
> +  @param[in]      InstructionData  Instruction parsing context
> +
> +  @retval 0                        Event handled successfully
> +  @retval Others                   New exception value to propagate
> +
> +**/
> +STATIC
> +UINT64
> +MsrExit (
> +  IN OUT GHCB                     *Ghcb,
> +  IN OUT EFI_SYSTEM_CONTEXT_X64   *Regs,
> +  IN     SEV_ES_INSTRUCTION_DATA  *InstructionData
> +  )
> +{
> +  UINT64  ExitInfo1, Status;
> +
> +  ExitInfo1 = 0;
> +
> +  switch (*(InstructionData->OpCodes + 1)) {
> +  case 0x30: // WRMSR

This comment looks great!

> +    ExitInfo1 = 1;
> +    Ghcb->SaveArea.Rax = Regs->Rax;
> +    GhcbSetRegValid (Ghcb, GhcbRax);
> +    Ghcb->SaveArea.Rdx = Regs->Rdx;
> +    GhcbSetRegValid (Ghcb, GhcbRdx);
> +    /* Fallthrough */

(1) This comment is very appreciated (I vaguely remember that the coding
style actually requires it), but we're supposed to put it like this:

    //
    // fall through
    //

(See: "git grep -B1 -A1 -i 'fall through'".)

> +  case 0x32: // RDMSR
> +    Ghcb->SaveArea.Rcx = Regs->Rcx;
> +    GhcbSetRegValid (Ghcb, GhcbRcx);
> +    break;
> +  default:
> +    return UnsupportedExit (Ghcb, Regs, InstructionData);
> +  }
> +
> +  Status = VmgExit (Ghcb, SVM_EXIT_MSR, ExitInfo1, 0);
> +  if (Status) {

(2) As usual, please check (Status > 0) or (Status != 0) explicitly.

Acked-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo

> +    return Status;
> +  }
> +
> +  if (!ExitInfo1) {
> +    if (!GhcbIsRegValid (Ghcb, GhcbRax) ||
> +        !GhcbIsRegValid (Ghcb, GhcbRdx)) {
> +      return UnsupportedExit (Ghcb, Regs, InstructionData);
> +    }
> +    Regs->Rax = Ghcb->SaveArea.Rax;
> +    Regs->Rdx = Ghcb->SaveArea.Rdx;
> +  }
> +
> +  return 0;
> +}
> +
>  #define IOIO_TYPE_STR       (1 << 2)
>  #define IOIO_TYPE_IN        1
>  #define IOIO_TYPE_INS       (IOIO_TYPE_IN | IOIO_TYPE_STR)
> @@ -743,6 +802,10 @@ VmgExitHandleVc (
>      NaeExit = IoioExit;
>      break;
>  
> +  case SVM_EXIT_MSR:
> +    NaeExit = MsrExit;
> +    break;
> +
>    default:
>      NaeExit = UnsupportedExit;
>    }
>