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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 7 Feb 2018 09:05:03 -0800
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Subject: Re: [PATCH tip-pti 2/2] x86/entry: interleave XOR register clearing
 with PUSH/MOV instructions
To: Andi Kleen <ak@linux.intel.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>,
        Ingo Molnar <mingo@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Wed, Feb 7, 2018 at 7:18 AM, Andi Kleen <ak@linux.intel.com> wrote:
>
> Fast path saves more than just register saving.  I changed the fast path
> to save all registers in my earlier clearregs branches

I know. I saw your patches. And I went "Eww".

> It is still quite a bit faster than all the slow stuff the C do_syscall
> code does (e.g. reloading all the arguments, setting up unnecessary
> frame pointers etc.).
>
> Just take a look at the disassembly of that function. It's really
> not very optimized.

Actually, only the argument reloading _really_ annoys me in do_syscall().

And I do think we should be able to fix that fairly easily by moving
it into the SYSCALLx() macros - at least for 64-bit (32-bit has nasty
issues with 64-bit arguments).

The other thing we need to do is to just pass down the system call
number as an argument instead of reloading it.

         Linus