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Subject[PATCH] KVM: X86: Remove stale values from ctxt->memop before emulation
When instruction decoding begins there could be stale values
in the ctxt->memop structure. This causes problems when an
instruction is emulated with more op-bytes then the guest
wants (like the bsr instruction which is always emulated
with 4 or 8 op-bytes).

The stale value in this structure causes the unit-test for
the bsrw instruction to fail. Initialize the memop.val with
0 to prevent such bugs (an alternative fix could be to
always emulate instructions with the number of op-bytes
requested by the guest).

Signed-off-by: Joerg Roedel <>
arch/x86/kvm/emulate.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index d4bf50c..1b516ec 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3937,6 +3937,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
struct opcode opcode;

ctxt->memop.type = OP_NONE;
+ ctxt->memop.val = 0;
ctxt->memopp = NULL;
ctxt->_eip = ctxt->eip;
ctxt->fetch.start = ctxt->_eip;

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