linux-kernel - Re: context tracking vs. syscall_trace_leave & do_notify

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Message-ID: <CALCETrVWAgsvhGYXitnacRurvYiPQO_c5V6kgmu2XR+y-LbgsQ@mail.gmail.com>
Date:	Fri, 1 May 2015 09:05:25 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Fr馘駻ic Weisbecker <fweisbec@...il.com>,
	X86 <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: context tracking vs. syscall_trace_leave & do_notify_resume loop

On Fri, May 1, 2015 at 9:00 AM, Rik van Riel <riel@...hat.com> wrote:
> On 05/01/2015 11:55 AM, Andy Lutomirski wrote:
>> On Thu, Apr 30, 2015 at 6:30 PM, Rik van Riel <riel@...hat.com> wrote:
>
>>> I suspect it would be possible to stick a call to a new function
>>> (return_to_user ?) right after the DISABLE_INTERRUPTS below, which
>>> could be used to do the context tracking user_enter just once, and
>>> later on also to load the user FPU context (patches I have sitting
>>> around).
>>>
>>> syscall_return:
>>>         /* The IRETQ could re-enable interrupts: */
>>>         DISABLE_INTERRUPTS(CLBR_ANY)
>>>         TRACE_IRQS_IRETQ
>>>
>>> Andy, Denys, do you guys see any issues with that idea?
>>
>> Ick.  Let's make the mess better before we make it worse.  Now that
>> Denys disentangled the syscall exit path from the interrupt exit path,
>> let me see if I can just rewrite the syscall exit path entirely later
>> this week.
>
> I suspect we probably only need two possible function
> calls at syscall exit time:
>
> 1) A function that is called with interrupts still
>    enabled, testing flags that could be set again
>    if something happens (eg. preemption) between
>    when the function is called, and we return to
>    user space.
>
> 2) A function that is called after the point of
>    no return, with interrupts disabled, which
>    does (mostly) small things that only happen
>    once.

I think we only need one function.  It would be (asm pseudocode):

disable irqs;
if (slow) {
  save extra regs;
  call function;
  restore extra regs;
}

return via opportunistic sysret path.

I can't see any legitimate reason for the current mess, except that
it's no complicated and so poorly documented that everyone's afraid of
fixing it.

--Andy
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