Re: [PATCH v2 22/36] x86/entry: Add C code for fast system call entries

[Andy Lutomirski]

On Tue, Oct 6, 2015 at 1:29 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Oct 6, 2015 at 9:25 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>> +       local_irq_enable();
>>> +       if (get_user(*(u32 *)&regs->cx,
>>> +                    (u32 __user __force *)(unsigned long)(u32)regs->sp)) {
>> ...
>>> +       local_irq_disable();
>>
>> this is expensive. Since we now do it in C code and can easily do
>> this, why does the code not do this all with interrupts disabled,
>> which is valid for user accesses but disables page faults, and then in
>> the unlikely situation where that fails, we do it the slow and careful
>> way?
>
> Ok. I notice that then a later patch removes the local_irq_disable()
> and calls do_syscall_32_irqs_on().
>
> So I guess that "just run get_user with interrupts disabled"
> optimization is pointless, because we'll just end up enabling
> interrupts at some point anyway, and it can just be done before the
> get_user().
>
> So never mind.
>

I'll improve the changelog.  This is all clear in my head, but I could
certainly describe it better.

I tried to structure this as much as possible as simple unoptimized
changes that would be easyish to understand from a correctness
perspective and then to optimize at the end.

--Andy
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