linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Message-ID: <CAK1hOcOZiLSG00uoAm_z0qxefLWiqD_WCsAWmtLB1VSvzPz-jg@mail.gmail.com>
Date:	Sat, 25 Apr 2015 04:17:04 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	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>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Fri, Apr 24, 2015 at 10:50 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Fri, Apr 24, 2015 at 1:46 PM, Denys Vlasenko
>>> This might be way more trouble than it's worth.
>>
>> Exactly my feeling. What are you trying to save? About four CPU
>> cycles of checking %ss != __KERNEL_DS on each switch_to?
>> That's not worth bothering about. Your last patch seems to be perfect.
>
> We'll have to do the write to ss almost every time an AMD CPU sleeps
> in a syscall.

Why do you think so?
Scheduling from a syscall which decided to block won't require
writing to %ss, since in this case %ss isn't NULL.

Writing to %ss will happen every time we schedule from an interrupt.
With timer interrupt every 1 ms it means scheduling at most ~1000
times per second, if _every_ such interrupt causes task switch.
This is still not often enough to worry.
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