KVM: x86: Support APX feature for guests

[PATCH RFC v1 15/20] KVM: x86: Reject EVEX-prefix instructions in the emulator

Posted by Chang S. Bae 3 months ago

Explicitly mark EVEX-prefixed opcodes (0x62) as unsupported, clarifying
current decoding behavior.

While new prefixes like REX2 extend GPR handling, EVEX emulation should
be addressed separately once after VEX support is implemented.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
---
 arch/x86/kvm/emulate.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 03f8e007b14e..9bd61ea496e5 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4952,8 +4952,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
 	if (ctxt->d & ModRM)
 		ctxt->modrm = insn_fetch(u8, ctxt);
 
-	/* vex-prefix instructions are not implemented */
-	if (ctxt->opcode_len == 1 && (ctxt->b == 0xc5 || ctxt->b == 0xc4) &&
+	/* VEX and EVEX-prefixed instructions are not implemented */
+	if (ctxt->opcode_len == 1 && (ctxt->b == 0xc5 || ctxt->b == 0xc4 || ctxt->b == 0x62) &&
 	    (mode == X86EMUL_MODE_PROT64 || (ctxt->modrm & 0xc0) == 0xc0)) {
 		ctxt->d = NotImpl;
 	}
-- 
2.51.0

Re: [PATCH RFC v1 15/20] KVM: x86: Reject EVEX-prefix instructions in the emulator

Posted by Paolo Bonzini 2 months, 4 weeks ago

On 11/10/25 19:01, Chang S. Bae wrote:
> Explicitly mark EVEX-prefixed opcodes (0x62) as unsupported, clarifying
> current decoding behavior.
> 
> While new prefixes like REX2 extend GPR handling, EVEX emulation should
> be addressed separately once after VEX support is implemented.
> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> ---
>   arch/x86/kvm/emulate.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 03f8e007b14e..9bd61ea496e5 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4952,8 +4952,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
>   	if (ctxt->d & ModRM)
>   		ctxt->modrm = insn_fetch(u8, ctxt);
>   
> -	/* vex-prefix instructions are not implemented */
> -	if (ctxt->opcode_len == 1 && (ctxt->b == 0xc5 || ctxt->b == 0xc4) &&
> +	/* VEX and EVEX-prefixed instructions are not implemented */
> +	if (ctxt->opcode_len == 1 && (ctxt->b == 0xc5 || ctxt->b == 0xc4 || ctxt->b == 0x62) &&
>   	    (mode == X86EMUL_MODE_PROT64 || (ctxt->modrm & 0xc0) == 0xc0)) {
>   		ctxt->d = NotImpl;
>   	}

VEX support is coming (will post tomorrow I think) so the patches around 
decode are going to need changes, but nothing major.

Paolo

Re: [PATCH RFC v1 15/20] KVM: x86: Reject EVEX-prefix instructions in the emulator

Posted by Chang S. Bae 2 months, 3 weeks ago

On 11/11/2025 8:37 AM, Paolo Bonzini wrote:
> 
> VEX support is coming (will post tomorrow I think) so the patches around 
> decode are going to need changes, but nothing major.

Sure, thanks for the note!