On Fri, May 1, 2015 at 9:00 AM, Rik van Riel <r...@redhat.com> wrote:
> On 05/01/2015 11:55 AM, Andy Lutomirski wrote:
>> On Thu, Apr 30, 2015 at 6:30 PM, Rik van Riel <r...@redhat.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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