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KVM: x86: Wrong emulation on 'xadd X, X' 

The emulator does not emulate the xadd instruction correctly if the two
operands are the same.  In this (unlikely) situation the result should be the
sum of X and X (2X) when it is currently X.  The solution is to first perform
writeback to the source, before writing to the destination.  The only
instruction which should be affected is xadd, as the other instructions that
perform writeback to the source use the extended accumlator (e.g., RAX:RDX).

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Nadav Amit 2014-06-15 16:12:58 +03:00 committed by Paolo Bonzini
parent 7dec5603b6
commit ee212297cd
1 changed files with 5 additions and 5 deletions
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10
arch/x86/kvm/emulate.c
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@ -4736,17 +4736,17 @@ special_insn:
goto done;
writeback:
if (!(ctxt->d & NoWrite)) {
rc = writeback(ctxt, &ctxt->dst);
if (rc != X86EMUL_CONTINUE)
goto done;
}
if (ctxt->d & SrcWrite) {
BUG_ON(ctxt->src.type == OP_MEM || ctxt->src.type == OP_MEM_STR);
rc = writeback(ctxt, &ctxt->src);
if (rc != X86EMUL_CONTINUE)
goto done;
}
if (!(ctxt->d & NoWrite)) {
rc = writeback(ctxt, &ctxt->dst);
if (rc != X86EMUL_CONTINUE)
goto done;
}
/*
* restore dst type in case the decoding will be reused