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

[Linus Torvalds]

On Wed, Feb 7, 2018 at 9:05 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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