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From: Denys Vlasenko <vda.linux@googlemail.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	X86 ML <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath
Date: Tue, 10 Mar 2015 14:50:18 +0100	[thread overview]
Message-ID: <CAK1hOcMTRhO3hhqD0Ft6qJ-NYn9uZ7tcTKxJB1=nRR-3fkbYEw@mail.gmail.com> (raw)
In-Reply-To: <20150310132147.GB26185@gmail.com>

On Tue, Mar 10, 2015 at 2:21 PM, Ingo Molnar <mingo@kernel.org> wrote:
>> Since this patch does add two extra MOVs,
>> I did benchmark these patches. They add exactly one cycle
>> to system call code path on my Sandy Bridge CPU.
>
> Hm, but that's the wrong direction, we should try to make it faster,
> and to clean it up

As you know, goals of "faster" and "cleaner" are often mutually exclusive.
C'est la vie :(

entry.S seem to lean towards "faster" to the extent where it became
a tangled maze of obscure optimizations.

Such as the mysterious, and not at all obvious existence of 8 byte
"safety padding" at the top of the 32-bit kernel stack. Before Andy
stumbled into it, it was not at all obvious that it is even there.
I had to write a test patch to verify it.
There is a long-standing latent bug in the code where this padding
is not accounted for.

> - but making it slower without really good reasons isn't good.

The thinking here is that cleaning up entry.S is a good reason.

We won't do anything which would slow it down by, say, 5%,
but one cycle may be considered acceptable loss.

  parent reply	other threads:[~2015-03-10 13:50 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-09 18:39 [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Denys Vlasenko
2015-03-09 18:39 ` [PATCH 1/4] x86: save r11 into pt_regs->flags on SYSCALL64 fastpath Denys Vlasenko
2015-03-09 20:02   ` Andy Lutomirski
2015-03-16 12:04   ` [tip:x86/asm] x86/asm/entry/64: Save R11 into pt_regs-> flags " tip-bot for Denys Vlasenko
2015-03-09 18:39 ` [PATCH 3/4] x86: save user rsp in pt_regs->sp " Denys Vlasenko
2015-03-09 20:11   ` Andy Lutomirski
2015-03-09 20:32     ` Denys Vlasenko
2015-03-09 20:43       ` Andy Lutomirski
2015-03-10 12:51   ` Ingo Molnar
2015-03-10 13:10     ` Andy Lutomirski
2015-03-10 13:18     ` Denys Vlasenko
2015-03-10 13:20       ` Andy Lutomirski
2015-03-10 13:26         ` Ingo Molnar
2015-03-10 13:21       ` Ingo Molnar
2015-03-10 13:26         ` Andy Lutomirski
2015-03-10 14:00           ` Denys Vlasenko
2015-03-10 14:02             ` Andy Lutomirski
2015-03-10 14:09               ` Denys Vlasenko
2015-03-10 13:28         ` Ingo Molnar
2015-03-10 13:50         ` Denys Vlasenko [this message]
2015-03-16  9:44           ` Ingo Molnar
2015-03-16 12:05   ` [tip:x86/asm] x86/asm/entry/64: Save user RSP in pt_regs-> sp " tip-bot for Denys Vlasenko
2015-03-10  6:00 ` [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Ingo Molnar

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