linux-kernel - Re: [PATCH 4/5] KVM: x86: Wrong register masking in 64-bit mode

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Message-ID: <536B7846.7080102@gmail.com>
Date:	Thu, 08 May 2014 15:27:50 +0300
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Nadav Amit <namit@...technion.ac.il>, mtosatti@...hat.com,
	hpa@...or.com
CC:	gleb@...nel.org, 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: Wrong register masking in 64-bit mode

On 5/7/14, 6:52 PM, Paolo Bonzini wrote:
> Il 07/05/2014 14:32, Nadav Amit ha scritto:
>> 32-bit operations are zero extended in 64-bit mode. Currently, the
>> code does
>> not handle them correctly and keeps the high bits. In 16-bit mode, the
>> high
>> 32-bits are kept intact.
>>
>> In addition, although it is not well-documented, when address override
>> prefix
>> is used with REP-string instruction, RCX high half is zeroed even if
>> ECX was
>> zero on the first iteration (as if an assignment was performed to ECX).
>
> Is this true even for REPZ and ZF=0 or REPNZ and ZF=1?
>
> Paolo
The REPZ and REPNZ condition is checked on the end of an iteration (see 
the REP instruction description on the SDM), so it does not matter. So 
even REPZ/RENZ would zero RCX high half.

This "feature" is not well-documented, but can be observed. Here is a 
small code you can try.

---
#include <stdio.h>
unsigned long src, dst;
int main()
{
	unsigned long long rsi, rdi, rcx;
	rcx = 0xffffffff00000000ull;
	rsi = (unsigned long long)&src | 0xffffffff00000000ull;
	rdi = (unsigned long long)&dst | 0xffffffff00000000ull;
	printf("before: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx);
	asm volatile (  ".byte 0x67\n\t"
			"repne cmpsd\n\t"
			: "+S" (rsi), "+D" (rdi), "+c" (rcx)
			: : "memory", "cc" );
	printf("after: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx);
	return 0;
}
---

Nadav

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