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Date:	Mon, 16 Jun 2014 13:38:51 -0400
From:	Bandan Das <bsd@...hat.com>
To:	Nadav Amit <namit@...technion.ac.il>
Cc:	pbonzini@...hat.com, 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'

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.

>  		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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