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Message-ID: <20150110212657.GE12218@pd.tnic>
Date: Sat, 10 Jan 2015 22:26:57 +0100
From: Borislav Petkov <bp@...en8.de>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Denys Vlasenko <vda.linux@...glemail.com>,
Denys Vlasenko <dvlasenk@...hat.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.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 01:08:33PM -0800, Linus Torvalds wrote:
> It was true for some AMD CPU's in particular. One insn/cycle vs two.
Probably on K8: Agner Fog's insn tables show reciprocal throughput of
1/2 for MOV r64/m64 vs 1 for PUSH/POP.
> I personally would be very happy to go back to push/pop sequences.
> Even without a fancy stack engine like Intel has done for a while,
> even *simple* cores can generally pair pushes and pops. I think the
I think all the modern x86 machines have stack engines now :-)
> original Pentium already had a special magic pairing logic to pair
> pushes and pops despite both instructions using %esp. It's a common
> and fairly trivial special case, and the fact that a few AMD
> microarchitectures didn't do it is likely not really a good reason to
> avoid repeated push/pop instructions.
Well, according to the optimization manual, on F15h (Bulldozer and
later) PUSH/POP are faster than MOVs and on F16h (Jaguar and later) both
MOV and PUSH/POP have latency of 1, with MOV having a 1/2 throughput vs
PUSH/POP throughput of 1. So theoretically we can do 2 MOVs per cycle
there vs 1 PUSH/POP.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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