linux-kernel - Re: [PATCH tip-pti 2/2] x86/entry: interleave XOR register clearing with PUSH/MOV instructions

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Message-ID: <CA+55aFxJri=_txLWBkVqs6peL5BLC0XpJSZUQr4YYGHC1s09mg@mail.gmail.com>
Date:   Wed, 7 Feb 2018 09:37:53 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Ingo Molnar <mingo@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH tip-pti 2/2] x86/entry: interleave XOR register clearing
 with PUSH/MOV instructions

On Wed, Feb 7, 2018 at 9:05 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The other thing we need to do is to just pass down the system call
> number as an argument instead of reloading it.

.. we may also want to disable some debug things.

For example, if you enable KASAN, it does insane things for
do_syscall(). I realize that nobody is supposed to care about
performance for KASAN, but still...

So one of the issues with do_syscall() is simply that _because_ it was
the rare case only, the code simply was never even looked at. But if y
ou look at the straight-line case, do_syscall_64() really doesn't look
too bad apart from the argument reloading that I do think is largely
fixable.

              Linus