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

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Date:	Thu, 23 Apr 2015 15:38:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Thu, Apr 23, 2015 at 3:31 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/23/2015 03:29 PM, Andy Lutomirski wrote:
>>>
>>> Yes, the NULL SS is a special thing in 64-bit mode.  I agree that
>>> context-switching it is probably the way to go; it should be cheap
>>> enough.  We might even be able to conditionalize it on an X86_BUG_ flag.
>>
>> I still don't see why context switches are a better place than just
>> before sysret, but I could be convinced.
>>
>
> Because there are way more sysrets than context switches, and Linux is
> particularly sensitive to system call latency, by design.

I mean sysret but only when SS might be zero.  Denys' approach
apparently needs ~4 cycles to check that (not bad), we could (yuck)
set a ti flag on context switch.

But yes, maybe you're right.

--Andy
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