linux-kernel - Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops

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Date:	Wed, 23 Apr 2014 14:01:26 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Nadav Amit <nadav.amit@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...nel.org>,
	Nadav Amit <namit@...technion.ac.il>, pbonzini@...hat.com,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected
 by string ops

On 04/23/2014 01:53 PM, Nadav Amit wrote:
>>
>> Err, operand size is forced to 64-bits, not address size.
>>
>> "The following aspects of near branches are controlled by the effective
>> operand size:
>>   • Truncation of the size of the instruction pointer"
>>
>> Still, "67h call" should not truncate EIP (which your patch does).
>>
> Yes, I missed it.
> But if I am not mistaken again, it means that the existing
> implementation of jmp_rel is broken as well when address-size override
> prefix is used. In this case, as I see it, the existing masking would
> cause the carry from the add operation to the lower half of the rip not
> to be added to the rip higher half.
> 
> I guess another patch is needed for that as well.
> 

Yes, on x86 JMP really should be thought of as "MOV ...,IP/EIP/RIP".  On
some other architectures, e.g. m68k, JMP acts as if it was
"LEA ...,PC", which causes some serious confusion for people familiar
with that model.  However, on x86 considering JMP as a MOV to the IP
register really is very consistent and will give you the right mental model.

	-hpa


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