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Date:   Thu,  2 Nov 2017 01:31:37 -0700
From:   Wanpeng Li <>
Cc:     Paolo Bonzini <>,
        Radim Krčmář <>,
        Wanpeng Li <>,
        Nadav Amit <>,
        Pedro Fonseca <>
Subject: [PATCH v3 1/3] KVM: X86: Fix operand size during instruction decoding

From: Wanpeng Li <>

Pedro reported:
  During tests that we conducted on KVM, we noticed that executing a "PUSH %ES"
  instruction under KVM produces different results on both memory and the SP
  register depending on whether EPT support is enabled. With EPT the SP is
  reduced by 4 bytes (and the written value is 0-padded) but without EPT support
  it is only reduced by 2 bytes. The difference can be observed when the CS.DB
  field is 1 (32-bit) but not when it's 0 (16-bit).

The internal segment descriptor cache exist even in real/vm8096 mode. The CS.D
also should be respected instead of just default operand-size/66H prefix during
instruction decoding. This patch fixes it by also adjusting operand-size according
to CS.D.

Reported-by: Pedro Fonseca <>
Tested-by: Pedro Fonseca <>
Cc: Paolo Bonzini <>
Cc: Radim Krčmář <>
Cc: Nadav Amit <>
Cc: Pedro Fonseca <>
Signed-off-by: Wanpeng Li <>
v2 -> v3:
 * cleanup the codes 
v1 -> v2:
 * respect cs.d for real/vm8096, other modes have already 
   been considered in init_emulate_ctxt().

 arch/x86/kvm/emulate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8079d14..6ebc4cb 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5000,6 +5000,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	bool op_prefix = false;
 	bool has_seg_override = false;
 	struct opcode opcode;
+	u16 dummy;
+	struct desc_struct desc;
 	ctxt->memop.type = OP_NONE;
 	ctxt->memopp = NULL;
@@ -5020,6 +5022,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	case X86EMUL_MODE_VM86:
 	case X86EMUL_MODE_PROT16:
 		def_op_bytes = def_ad_bytes = 2;
+		if (mode < X86EMUL_MODE_PROT16) {
+			ctxt->ops->get_segment(ctxt, &dummy, &desc, NULL, VCPU_SREG_CS);
+			if (desc.d)
+				def_op_bytes = 4;
+		}
 	case X86EMUL_MODE_PROT32:
 		def_op_bytes = def_ad_bytes = 4;

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