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Message-ID: <CALCETrXjoPS1keU6p3nnZh8P-xzZBFwao+44272VOwW2TabThw@mail.gmail.com> Date: Tue, 6 Oct 2015 11:25:25 -0700 From: Andy Lutomirski <luto@...capital.net> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Brian Gerst <brgerst@...il.com>, Denys Vlasenko <dvlasenk@...hat.com>, Borislav Petkov <bp@...en8.de> Subject: Re: [PATCH v2 22/36] x86/entry: Add C code for fast system call entries On Tue, Oct 6, 2015 at 1:29 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Tue, Oct 6, 2015 at 9:25 AM, Linus Torvalds > <torvalds@...ux-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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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