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

Bandan Das
2014-06-17T15:35:24Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> Il 16/06/2014 19:38, Bandan Das ha scritto:
>> 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.
>
> Sure, please post a patch and I'll apply it right away.

Well, I meant it more as a review and "suggested changes" to this patchset
Nadav posted, but yeah, if you prefer, I can post a change myself. I will
make a pass through other uses of BUG() in the code too.

> Paolo
>
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