From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: 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@kernel.org, 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 14:21:48 +0100 [thread overview]
Message-ID: <20150310132147.GB26185@gmail.com> (raw)
In-Reply-To: <54FEEF0D.5080505@redhat.com>
* Denys Vlasenko <dvlasenk@redhat.com> wrote:
> > So there are now +2 instructions (5 instead of 3) in the
> > system_call path, but there are -2 instructions in the SYSRETQ
> > path,
>
> Unfortunately, no. [...]
So I assumed that it was an equivalent transformation, given that none
of the changelogs spelled out the increase in overhead ...
> [...] There is only this change in SYSRETQ path, which simply
> changes where we get RSP from:
>
> @@ -293,7 +289,7 @@ ret_from_sys_call:
> CFI_REGISTER rip,rcx
> movq EFLAGS(%rsp),%r11
> /*CFI_REGISTER rflags,r11*/
> - movq PER_CPU_VAR(old_rsp), %rsp
> + movq RSP(%rsp),%rsp
> /*
> * 64bit SYSRET restores rip from rcx,
> * rflags from r11 (but RF and VM bits are forced to 0),
>
> Most likely, no change in execution speed here.
> At best, it is one cycle faster somewhere in address generation unit
> because for PER_CPU_VAR() address evaluation, GS base is nonzero.
>
> 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 - but making it slower without really good reasons
isn't good.
Is 'usersp' really that much of a complication?
Thanks,
Ingo
next prev parent reply other threads:[~2015-03-10 13:21 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 [this message]
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
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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