Re: [PATCH] x86/sev-es: Fix SEV-ES OUT/IN immediate opcode vc handling

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 16, 2020, Peter Gonda wrote:
> 
> The IN and OUT immediate instructions only use an 8-bit immediate. The
> current VC handler uses the entire 32-bit immediate value. These
> instructions only set the first bytes.
> 
> Tested with a loop back port with "outb %0,$0xe0". Before the port seen
> by KVM was 0xffffffffffffffe0 instead of 0xe0. After the correct port
> was seen by KVM and the guests loop back OUT then IN were equal.
> 
> 
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Acked-by: David Rientjes <rientjes@google.com>
> 
> 
> ---
>  arch/x86/kernel/sev-es-shared.c | 8 ++++++--
>  drivers/Makefile                | 1 +
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
> index 7d04b356d44d..6c790377c55c 100644
> --- a/arch/x86/kernel/sev-es-shared.c
> +++ b/arch/x86/kernel/sev-es-shared.c
> @@ -305,14 +305,14 @@ static enum es_result vc_ioio_exitinfo(struct es_em_ctxt *ctxt, u64 *exitinfo)
>  	case 0xe4:
>  	case 0xe5:
>  		*exitinfo |= IOIO_TYPE_IN;
> -		*exitinfo |= (u64)insn->immediate.value << 16;
> +		*exitinfo |= insn->immediate.bytes[0] << 16;

Can't we just drop the explicit cast to u64?  Or explicitly cast to u8?  Doesn't
really matter, but poking into the backing bytes feels a bit backwards.

>  		break;
>  
>  	/* OUT immediate opcodes */
>  	case 0xe6:
>  	case 0xe7:
>  		*exitinfo |= IOIO_TYPE_OUT;
> -		*exitinfo |= (u64)insn->immediate.value << 16;
> +		*exitinfo |= insn->immediate.bytes[0] << 16;
>  		break;