Commit 825f0aec authored by Chang S. Bae's avatar Chang S. Bae Committed by Sean Christopherson
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KVM: x86: Refactor REX prefix handling in instruction emulation 



Restructure how to represent and interpret REX fields, preparing
for handling of both REX2 and VEX.

REX uses the upper four bits of a single byte as a fixed identifier,
and the lower four bits containing the data. VEX and REX2 extends this so
that the first byte identifies the prefix and the rest encode additional
bits; and while VEX only has the same four data bits as REX, eight zero
bits are a valid value for the data bits of REX2.  So, stop storing the
REX byte as-is.  Instead, store only the low bits of the REX prefix and
track separately whether a REX-like prefix was used.

No functional changes intended.

Signed-off-by: default avatarChang S. Bae <chang.seok.bae@intel.com>
Message-ID: <20251110180131.28264-11-chang.seok.bae@intel.com>
[Extracted from APX series; removed bitfields and REX2-specific default. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Link: https://patch.msgid.link/20251114003633.60689-9-pbonzini@redhat.com


[sean: name REX_{BXRW} enum "rex_bits"]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 4cb21be4

arch/x86/kvm/emulate.c

+21 −12
Original line number Diff line number Diff line
@@ -239,6 +239,13 @@ enum x86_transfer_type { 
	X86_TRANSFER_TASK_SWITCH,

};



enum rex_bits {

	REX_B = 1,

	REX_X = 2,

	REX_R = 4,

	REX_W = 8,

};



static void writeback_registers(struct x86_emulate_ctxt *ctxt)

{

	unsigned long dirty = ctxt->regs_dirty;
@@ -919,7 +926,7 @@ static void *decode_register(struct x86_emulate_ctxt *ctxt, u8 modrm_reg, 
			     int byteop)

{

	void *p;

	int highbyte_regs = (ctxt->rex_prefix == 0) && byteop;

	int highbyte_regs = (ctxt->rex_prefix == REX_NONE) && byteop;



	if (highbyte_regs && modrm_reg >= 4 && modrm_reg < 8)

		p = (unsigned char *)reg_rmw(ctxt, modrm_reg & 3) + 1;
@@ -1110,7 +1117,7 @@ static void decode_register_operand(struct x86_emulate_ctxt *ctxt, 
	if (ctxt->d & ModRM)

		reg = ctxt->modrm_reg;

	else

		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);

		reg = (ctxt->b & 7) | (ctxt->rex_bits & REX_B ? 8 : 0);



	__decode_register_operand(ctxt, op, reg);

}
@@ -1129,9 +1136,9 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt, 
	int rc = X86EMUL_CONTINUE;

	ulong modrm_ea = 0;



	ctxt->modrm_reg = ((ctxt->rex_prefix << 1) & 8); /* REX.R */

	index_reg = (ctxt->rex_prefix << 2) & 8; /* REX.X */

	base_reg = (ctxt->rex_prefix << 3) & 8; /* REX.B */

	ctxt->modrm_reg = (ctxt->rex_bits & REX_R ? 8 : 0);

	index_reg = (ctxt->rex_bits & REX_X ? 8 : 0);

	base_reg = (ctxt->rex_bits & REX_B ? 8 : 0);



	ctxt->modrm_mod = (ctxt->modrm & 0xc0) >> 6;

	ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3;
@@ -2464,7 +2471,7 @@ static int em_sysexit(struct x86_emulate_ctxt *ctxt) 


	setup_syscalls_segments(&cs, &ss);



	if ((ctxt->rex_prefix & 0x8) != 0x0)

	if (ctxt->rex_bits & REX_W)

		usermode = X86EMUL_MODE_PROT64;

	else

		usermode = X86EMUL_MODE_PROT32;
@@ -4850,7 +4857,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int 
		case 0x40 ... 0x4f: /* REX */

			if (mode != X86EMUL_MODE_PROT64)

				goto done_prefixes;

			ctxt->rex_prefix = ctxt->b;

			ctxt->rex_prefix = REX_PREFIX;

			ctxt->rex_bits   = ctxt->b & 0xf;

			continue;

		case 0xf0:	/* LOCK */

			ctxt->lock_prefix = 1;
@@ -4864,15 +4872,15 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int 
		}



		/* Any legacy prefix after a REX prefix nullifies its effect. */



		ctxt->rex_prefix = 0;

		ctxt->rex_prefix = REX_NONE;

		ctxt->rex_bits = 0;

	}



done_prefixes:



	/* REX prefix. */

	if (ctxt->rex_prefix & 8)

		ctxt->op_bytes = 8;	/* REX.W */

	if (ctxt->rex_bits & REX_W)

		ctxt->op_bytes = 8;



	/* Opcode byte(s). */

	if (ctxt->b == 0x0f) {
@@ -5138,7 +5146,8 @@ void init_decode_cache(struct x86_emulate_ctxt *ctxt) 
{

	/* Clear fields that are set conditionally but read without a guard. */

	ctxt->rip_relative = false;

	ctxt->rex_prefix = 0;

	ctxt->rex_prefix = REX_NONE;

	ctxt->rex_bits = 0;

	ctxt->lock_prefix = 0;

	ctxt->op_prefix = false;

	ctxt->rep_prefix = 0;

arch/x86/kvm/kvm_emulate.h

+10 −1
Original line number Diff line number Diff line
@@ -319,6 +319,14 @@ typedef void (*fastop_t)(struct fastop *); 
#define NR_EMULATOR_GPRS	8

#endif



/*

 * Distinguish between no prefix, REX, or in the future REX2.

 */

enum rex_type {

	REX_NONE,

	REX_PREFIX,

};



struct x86_emulate_ctxt {

	void *vcpu;

	const struct x86_emulate_ops *ops;
@@ -360,7 +368,8 @@ struct x86_emulate_ctxt { 
	int (*check_perm)(struct x86_emulate_ctxt *ctxt);



	bool rip_relative;

	u8 rex_prefix;

	enum rex_type rex_prefix;

	u8 rex_bits;

	u8 lock_prefix;

	u8 rep_prefix;

	/* bitmaps of registers in _regs[] that can be read */