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

[Linus Torvalds]

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.

            Linus
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