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

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Message-ID: <539FCF48.5060705@redhat.com>
Date:	Tue, 17 Jun 2014 07:16:56 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>, Nadav Amit <namit@...technion.ac.il>
CC:	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'

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.

Paolo

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