Messages in this thread | | | From | Bandan Das <> | Subject | Re: [PATCH 2/6] KVM: x86: Wrong emulation on 'xadd X, X' | Date | Mon, 16 Jun 2014 13:38:51 -0400 |
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Nadav Amit <namit@cs.technion.ac.il> writes:
> 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> > --- > arch/x86/kvm/emulate.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index f0b0a10..3c8d867 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -4711,17 +4711,17 @@ 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); While we are here, I think we should replace this BUG_ON with a warning and return X86EMUL_UNHANDLEABLE if the condition is true.
> 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
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