| Date | Tue, 6 Oct 2015 09:29:44 +0100 | Subject | Re: [PATCH v2 22/36] x86/entry: Add C code for fast system call entries | From | Linus Torvalds <> |
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On Tue, Oct 6, 2015 at 9:25 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > >> + local_irq_enable(); >> + if (get_user(*(u32 *)®s->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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