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

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Message-ID: <553977AA.9040802@zytor.com>
Date:	Thu, 23 Apr 2015 15:52:26 -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 03:38 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.

	-hpa


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