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

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Date:	Tue, 17 Jun 2014 11:35:24 -0400
From:	Bandan Das <bsd@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Nadav Amit <namit@...technion.ac.il>, gleb@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] KVM: x86: Wrong emulation on 'xadd X, X'

Paolo Bonzini <pbonzini@...hat.com> writes:

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