linux-kernel - Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

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Date:	Wed, 01 Apr 2015 13:10:57 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Brian Gerst <brgerst@...il.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

On 04/01/2015 12:21 AM, Brian Gerst wrote:
> On Tue, Mar 31, 2015 at 1:00 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> After TESTs, use logically correct JZ mnemonic instead of JE
>> (this doesn't change code).
>>
>> Tidy up CMPW insns:
>>
>> Modern CPUs are not good with 16-bit operations.
>> The instructions with 16-bit immediates are especially bad,
>> on many CPUs they cause length changing prefix stall
>> in the decoders, costing ~6 cycles to recover.
>>
>> Replace CMPWs with CMPLs.
>> Of these, for form with 8-bit sign-extended immediates
>> it is a win because they are smaller now
>> (no 0x66 prefix anymore);
>> ones with 16-bit immediates are faster.
>>
>> @@ -708,7 +708,7 @@ END(sysenter_badsys)
>>  #ifdef CONFIG_X86_ESPFIX32
>>         movl %ss, %eax
>>         /* see if on espfix stack */
>> -       cmpw $__ESPFIX_SS, %ax
>> +       cmpl $__ESPFIX_SS, %eax
>>         jne 27f
>>         movl $__KERNEL_DS, %eax
>>         movl %eax, %ds
> 
> This is incorrect.  32-bit reads from a segment register are not
> zero-extended.  The upper 16 bits are implementation-defined.  Most
> processors will clear them but it's not guaranteed.

I did not know that. I was sure they are always zero extended.


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