linux-kernel - Re: [PATCH 3/3] x86: get rid of FIXUP_TOP_OF_STACK/RESTORE_TOP_OF

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Message-ID: <CALCETrUhfatMLXQxa+YwLN-14Jbx0uPSpMRJ7a6quyCL_nEgEw@mail.gmail.com>
Date:	Wed, 18 Mar 2015 14:02:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86: get rid of FIXUP_TOP_OF_STACK/RESTORE_TOP_OF_STACK

On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> FIXUP_TOP_OF_STACK is only necessary because we don't save %r11
> to pt_regs->r11 on SYSCALL64 fast path, but we want ptrace to see
> it populated.
>
> Bite the bullet, add a single additional PUSH insn, and remove
> FIXUP_TOP_OF_STACK.
>
> RESTORE_TOP_OF_STACK is already a nop. Remove it too.
>
> On SandyBridge CPU, it does not get slower:
> measured 54.22 ns per getpid syscall before and after last two changes
> on defconfig kernel.

Hallelujah!

--Andy
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