linux-kernel - Re: [PATCH 9/9] KVM: x86: smsw emulation is incorrect in 64-bit mode

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Message-Id: <CB894343-EC15-4A5D-B461-B012FC19FC3A@gmail.com>
Date:	Thu, 5 Jun 2014 18:02:16 +0300
From:	Nadav Amit <nadav.amit@...il.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 9/9] KVM: x86: smsw emulation is incorrect in 64-bit mode


On Jun 5, 2014, at 5:53 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:

> Il 02/06/2014 17:34, Nadav Amit ha scritto:
>> In 64-bit mode, when the destination is a register, the assignment is done
>> according to the operand size. Otherwise (memory operand or no 64-bit mode), a
>> 16-bit assignment is performed.
> 
> I'm sorry, I'm missing the place where 64-bit mode is taken into account?
It is not, since on 32-bit mode the high-order 16 bits of a register destination are undefined. 
If I recall correctly, in this case the high-order 16-bits on native system actually reflect the high-order 16-bits of CR0.

Therefore, regardless to the mode of operation:
1. If the target is memory - the destination is 16-bit
2. If the target is a register - the register is written according to the operand size.

Nadav--
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