Re: [PATCH RFC v1 14/20] KVM: x86: Emulate REX2-prefixed 64-bit absolute jump

["Chang S. Bae" <chang.seok.bae@intel.com>]

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On 11/11/2025 8:39 AM, Paolo Bonzini wrote:
> 
> We can just add a NoRex bit and apply it to the six reows you touch in 
> patch 13.

Yeah, I think that is much simple and better. Attached is the diff for
this:

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diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index b2490e56cb71..5f36dbcec484 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -179,6 +179,7 @@
 #define TwoMemOp    ((u64)1 << 55)  /* Instruction has two memory operand */
 #define IsBranch    ((u64)1 << 56)  /* Instruction is considered a branch. */
 #define ShadowStack ((u64)1 << 57)  /* Instruction affects Shadow Stacks. */
+#define NoRex2      ((u64)1 << 58)  /* Instruction has no use of REX2 prefix */
 
 #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
 
@@ -4247,7 +4248,7 @@ static const struct opcode opcode_table[256] = {
 	/* 0x38 - 0x3F */
 	I6ALU(NoWrite, em_cmp), N, N,
 	/* 0x40 - 0x4F */
-	X8(I(DstReg, em_inc)), X8(I(DstReg, em_dec)),
+	X8(I(DstReg | NoRex2, em_inc)), X8(I(DstReg | NoRex2, em_dec)),
 	/* 0x50 - 0x57 */
 	X8(I(SrcReg | Stack, em_push)),
 	/* 0x58 - 0x5F */
@@ -4265,7 +4266,7 @@ static const struct opcode opcode_table[256] = {
 	I2bvIP(DstDI | SrcDX | Mov | String | Unaligned, em_in, ins, check_perm_in), /* insb, insw/insd */
 	I2bvIP(SrcSI | DstDX | String, em_out, outs, check_perm_out), /* outsb, outsw/outsd */
 	/* 0x70 - 0x7F */
-	X16(D(SrcImmByte | NearBranch | IsBranch)),
+	X16(D(SrcImmByte | NearBranch | IsBranch | NoRex2)),
 	/* 0x80 - 0x87 */
 	G(ByteOp | DstMem | SrcImm, group1),
 	G(DstMem | SrcImm, group1),
@@ -4289,15 +4290,15 @@ static const struct opcode opcode_table[256] = {
 	II(ImplicitOps | Stack, em_popf, popf),
 	I(ImplicitOps, em_sahf), I(ImplicitOps, em_lahf),
 	/* 0xA0 - 0xA7 */
-	I2bv(DstAcc | SrcMem | Mov | MemAbs, em_mov),
-	I2bv(DstMem | SrcAcc | Mov | MemAbs | PageTable, em_mov),
-	I2bv(SrcSI | DstDI | Mov | String | TwoMemOp, em_mov),
-	I2bv(SrcSI | DstDI | String | NoWrite | TwoMemOp, em_cmp_r),
+	I2bv(DstAcc | SrcMem | Mov | MemAbs | NoRex2, em_mov),
+	I2bv(DstMem | SrcAcc | Mov | MemAbs | PageTable | NoRex2, em_mov),
+	I2bv(SrcSI | DstDI | Mov | String | TwoMemOp | NoRex2, em_mov),
+	I2bv(SrcSI | DstDI | String | NoWrite | TwoMemOp | NoRex2, em_cmp_r),
 	/* 0xA8 - 0xAF */
-	I2bv(DstAcc | SrcImm | NoWrite, em_test),
-	I2bv(SrcAcc | DstDI | Mov | String, em_mov),
-	I2bv(SrcSI | DstAcc | Mov | String, em_mov),
-	I2bv(SrcAcc | DstDI | String | NoWrite, em_cmp_r),
+	I2bv(DstAcc | SrcImm | NoWrite | NoRex2, em_test),
+	I2bv(SrcAcc | DstDI | Mov | String | NoRex2, em_mov),
+	I2bv(SrcSI | DstAcc | Mov | String | NoRex2, em_mov),
+	I2bv(SrcAcc | DstDI | String | NoWrite | NoRex2, em_cmp_r),
 	/* 0xB0 - 0xB7 */
 	X8(I(ByteOp | DstReg | SrcImm | Mov, em_mov)),
 	/* 0xB8 - 0xBF */
@@ -4327,17 +4328,17 @@ static const struct opcode opcode_table[256] = {
 	/* 0xD8 - 0xDF */
 	N, E(0, &escape_d9), N, E(0, &escape_db), N, E(0, &escape_dd), N, N,
 	/* 0xE0 - 0xE7 */
-	X3(I(SrcImmByte | NearBranch | IsBranch, em_loop)),
-	I(SrcImmByte | NearBranch | IsBranch, em_jcxz),
-	I2bvIP(SrcImmUByte | DstAcc, em_in,  in,  check_perm_in),
-	I2bvIP(SrcAcc | DstImmUByte, em_out, out, check_perm_out),
+	X3(I(SrcImmByte | NearBranch | IsBranch | NoRex2, em_loop)),
+	I(SrcImmByte | NearBranch | IsBranch | NoRex2, em_jcxz),
+	I2bvIP(SrcImmUByte | DstAcc | NoRex2, em_in,  in,  check_perm_in),
+	I2bvIP(SrcAcc | DstImmUByte | NoRex2, em_out, out, check_perm_out),
 	/* 0xE8 - 0xEF */
-	I(SrcImm | NearBranch | IsBranch | ShadowStack, em_call),
-	D(SrcImm | ImplicitOps | NearBranch | IsBranch),
-	I(SrcImmFAddr | No64 | IsBranch, em_jmp_far),
-	D(SrcImmByte | ImplicitOps | NearBranch | IsBranch),
-	I2bvIP(SrcDX | DstAcc, em_in,  in,  check_perm_in),
-	I2bvIP(SrcAcc | DstDX, em_out, out, check_perm_out),
+	I(SrcImm | NearBranch | IsBranch | ShadowStack | NoRex2, em_call),
+	D(SrcImm | ImplicitOps | NearBranch | IsBranch | NoRex2),
+	I(SrcImmFAddr | No64 | IsBranch | NoRex2, em_jmp_far),
+	D(SrcImmByte | ImplicitOps | NearBranch | IsBranch | NoRex2),
+	I2bvIP(SrcDX | DstAcc | NoRex2, em_in,  in,  check_perm_in),
+	I2bvIP(SrcAcc | DstDX | NoRex2, em_out, out, check_perm_out),
 	/* 0xF0 - 0xF7 */
 	N, DI(ImplicitOps, icebp), N, N,
 	DI(ImplicitOps | Priv, hlt), D(ImplicitOps),
@@ -4378,12 +4379,12 @@ static const struct opcode twobyte_table[256] = {
 	N, GP(ModRM | DstMem | SrcReg | Mov | Sse, &pfx_0f_2b),
 	N, N, N, N,
 	/* 0x30 - 0x3F */
-	II(ImplicitOps | Priv, em_wrmsr, wrmsr),
-	IIP(ImplicitOps, em_rdtsc, rdtsc, check_rdtsc),
-	II(ImplicitOps | Priv, em_rdmsr, rdmsr),
-	IIP(ImplicitOps, em_rdpmc, rdpmc, check_rdpmc),
-	I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack, em_sysenter),
-	I(ImplicitOps | Priv | EmulateOnUD | IsBranch | ShadowStack, em_sysexit),
+	II(ImplicitOps | Priv | NoRex2, em_wrmsr, wrmsr),
+	IIP(ImplicitOps | NoRex2, em_rdtsc, rdtsc, check_rdtsc),
+	II(ImplicitOps | Priv | NoRex2, em_rdmsr, rdmsr),
+	IIP(ImplicitOps | NoRex2, em_rdpmc, rdpmc, check_rdpmc),
+	I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack | NoRex2, em_sysenter),
+	I(ImplicitOps | Priv | EmulateOnUD | IsBranch | ShadowStack | NoRex2, em_sysexit),
 	N, N,
 	N, N, N, N, N, N, N, N,
 	/* 0x40 - 0x4F */
@@ -4401,7 +4402,7 @@ static const struct opcode twobyte_table[256] = {
 	N, N, N, N,
 	N, N, N, GP(SrcReg | DstMem | ModRM | Mov, &pfx_0f_6f_0f_7f),
 	/* 0x80 - 0x8F */
-	X16(D(SrcImm | NearBranch | IsBranch)),
+	X16(D(SrcImm | NearBranch | IsBranch | NoRex2)),
 	/* 0x90 - 0x9F */
 	X16(D(ByteOp | DstMem | SrcNone | ModRM| Mov)),
 	/* 0xA0 - 0xA7 */
@@ -4888,7 +4889,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
 	if (ctxt->b == 0x0f) {
 		/* Escape byte: start two-byte opcode sequence */
 		ctxt->b = insn_fetch(u8, ctxt);
-		if (ctxt->b == 0x38) {
+		if (ctxt->b == 0x38 && ctxt->rex_prefix != REX2_PREFIX) {
 			/* Three-byte opcode */
 			ctxt->opcode_len = 3;
 			ctxt->b = insn_fetch(u8, ctxt);
@@ -4905,6 +4906,9 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
 	}
 	ctxt->d = opcode.flags;
 
+	if (ctxt->d & NoRex2 && ctxt->rex_prefix == REX2_PREFIX)
+		ctxt->d = Undefined;
+
 	if (ctxt->d & ModRM)
 		ctxt->modrm = insn_fetch(u8, ctxt);