linux-kernel - Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

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Date:	Thu, 23 Apr 2015 17:59:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

On 04/23/2015 03:55 PM, Andy Lutomirski wrote:
> On Thu, Apr 23, 2015 at 3:52 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 04/23/2015 03:38 PM, Andy Lutomirski wrote:
>>>>
>>>> Because there are way more sysrets than context switches, and Linux is
>>>> particularly sensitive to system call latency, by design.
>>>
>>
>> Just to clarify: why would Linux be more sensitive to system call by
>> design?  It enables much simpler APIs and avoids hacks like sending down
>> a syscall task list (which was genuinely proposed at one point.)  If
>> kernel entry/exit is too expensive, then the APIs get more complex
>> because they *have* to do everything in the smallest number of system calls.
>>
> 
> It's a matter of the ratio, right?  One cycle of syscall overhead
> saved is worth some number of context switch cycles added, and the
> ratio probably varies by workload.
> 

Correct.  For workloads which do *no* system calls it is kind of "special".

> If we do syscall, two context switches, and sysret, then we wouldn't
> have been better off fixing it on sysret.  But maybe most workloads
> still prefer the fixup on context switch.
> 

There is also a matter of latency, which tends to be more critical for
syscall.

	-hpa


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