linux-kernel - Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

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Date:   Mon, 5 Feb 2018 14:25:02 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall
 arguments for 64bit kernels

On Mon, Feb 5, 2018 at 2:22 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But I'm not timing it.

I lied.

I did this:

        for (i = 0; i < 100000; i++)
                asm(".rept 16384\n"
                "subq $128,%rsp\n\t"
                "pushq %rbx\n\t"
                "pushq %r10\n\t"
                "pushq %r11\n\t"
                "pushq %r12\n\t"
                "pushq %r13\n\t"
                "pushq %r14\n\t"
                "pushq %r15\n\t"

                "popq %r15\n\t"
                "popq %r14\n\t"
                "popq %r13\n\t"
                "popq %r12\n\t"
                "popq %r11\n\t"
                "popq %r10\n\t"
                "popq %rbx\n\t"
                "addq $128,%rsp\n\t"
                ".endr");

and then I timed it like that, and with "xorq" of the register after
each "pushq".

And the timings came out the same, to within the (bad) timing I did.

So I really do think you can just put the xor right next to the push,
and it will be effectively free.

            Linus