linux-kernel - Re: [PATCH 3/4] x86: open-code register save/restore in trace

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Date:	Sat, 10 Jan 2015 22:09:24 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 3/4] x86: open-code register save/restore in
 trace_hardirqs thunks

On Sat, Jan 10, 2015 at 10:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sat, Jan 10, 2015 at 12:42 PM, Borislav Petkov <bp@...en8.de> wrote:
>> On Sat, Jan 10, 2015 at 12:17:13PM -0800, Andy Lutomirski wrote:
>>> I asked this once, and someone told me that push/pop has lower
>>> throughput.  I find this surprising.
>>
>> Implicit dependency on %rsp probably. The MOVs allow you to start more
>> stuff out-of-order I'd guess...
>
> AIUI modern CPUs have fancy stack engines that match call/ret pairs,
> and presumably they can speculate rsp values across multiple pushes
> and pops very quickly.

Yes, stack engine hangs off the pipeline right after decode stage.

> Also, don't compilers generally use push and pop to save and restore
> callee-saved registers?  I think that function calls are common enough
> that the CPU vendors would have made these sequences fast.

Compilers can't predict which functions are hottest.
Using mov's would bloat prologues by about factor of 5.

I think using push/pop is okay. In the very hottest code paths
you may want to prefer mov's.
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