Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 6 Oct 2015 11:25:25 -0700 | Subject | Re: [PATCH v2 22/36] x86/entry: Add C code for fast system call entries |
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On Tue, Oct 6, 2015 at 1:29 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > 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. >
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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