Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, May 15, 2020 at 12:19:19PM -0400, Paolo Bonzini wrote:
> Instructions starting with 0f18 up to 0f1f are reserved nops, except those
> that were assigned to MPX.

Well, they're probably reserved NOPs again :-D.

> These include the endbr markers used by CET.

And RDSPP.  Wouldn't it make sense to treat RDSPP as a #UD even though it's
a NOP if CET is disabled?  The logic being that a sane guest will execute
RDSSP iff CET is enabled, and in that case it'd be better to inject a #UD
than to silently break the guest.

Extending that logic to future features, wouldn't it then make sense to
keep the current #UD behavior for all opcodes to avoid silently breakage?
I.e. change only the opcodes for endbr (which consume only 2 of 255 ModRMs
for 0f 1e) to be NOPs.

> List them correctly in the opcode table.
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/emulate.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index de5476f8683e..d0e2825ae617 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4800,8 +4800,12 @@ static const struct opcode twobyte_table[256] = {
>  	GP(ModRM | DstReg | SrcMem | Mov | Sse, &pfx_0f_10_0f_11),
>  	GP(ModRM | DstMem | SrcReg | Mov | Sse, &pfx_0f_10_0f_11),
>  	N, N, N, N, N, N,
> -	D(ImplicitOps | ModRM | SrcMem | NoAccess),
> -	N, N, N, N, N, N, D(ImplicitOps | ModRM | SrcMem | NoAccess),
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), /* 4 * prefetch + 4 * reserved NOP */
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), N, N,
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), /* 8 * reserved NOP */
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), /* 8 * reserved NOP */
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), /* 8 * reserved NOP */
> +	D(ImplicitOps | ModRM | SrcMem | NoAccess), /* NOP + 7 * reserved NOP */
>  	/* 0x20 - 0x2F */
>  	DIP(ModRM | DstMem | Priv | Op3264 | NoMod, cr_read, check_cr_read),
>  	DIP(ModRM | DstMem | Priv | Op3264 | NoMod, dr_read, check_dr_read),
> -- 
> 2.18.2
>