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

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Date:	Tue, 31 Mar 2015 18:21:42 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.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 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.

--
Brian Gerst
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