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

[Andi Kleen]

> Plus the fastpath couldn't clear those registers anyway, since it
> didn't even _save_ them - exactly because the whole point of the
> fastpath was that not all registers are clobbered by the calling
> conventions.

Fast path saves more than just register saving.  I changed the fast path
to save all registers in my earlier clearregs branches

https://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git/log/?h=spec/clearregs-3

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.

-Andi