Re: [PATCH 2/6] KVM: x86: Wrong emulation on 'xadd X, X'

[Paolo Bonzini]

Il 16/06/2014 19:38, Bandan Das ha scritto:
> Nadav Amit <namit@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> > ---
> >  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.

Sure, please post a patch and I'll apply it right away.

Paolo

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/