linux-kernel - Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers

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Message-ID: <CA+55aFweH7=igcXQ7AoC_m-kHM8srv85KAQyTLYj9D5oozNo3Q@mail.gmail.com>
Date:	Thu, 14 May 2015 10:05:11 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.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] x86/asm/entry/64: Use shorter MOVs from segmers registers

On Thu, May 14, 2015 at 9:55 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> "movw %ds,%cx" insn needs a 0x66 prefix, while "movw %ds,%ecx" does not.
> The difference is that it overwrite entire %ecx, not only its low half,
> but subsequent code doesn't depend on the value of top half of %ecx,
> we can safely use the shorter insn.

I don't object to the patch, but did we actually confirm that it
always overwrites all of %ecx? The segment move instructions are
schizofrenic when it comes to sizes, and sometimes write just 16 bits
even when the instruction size is 32.

Was that just for push and/or move-to-memory?

                       Linus
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