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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 23 Apr 2015 15:38:47 -0700
Message-ID: <CALCETrV2t1ynQ0KpRv71+_1wqUg6d2=k-guqHatrEHJLeJ9d6Q@mail.gmail.com>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Borislav Petkov <bp@alien8.de>, Denys Vlasenko <vda.linux@googlemail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        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>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Apr 23, 2015 at 3:31 PM, H. Peter Anvin <hpa@zytor.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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