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Date: Wed, 7 Feb 2018 07:18:54 -0800
From: Andi Kleen <ak@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: [PATCH tip-pti 2/2] x86/entry: interleave XOR register clearing
 with PUSH/MOV instructions
Message-ID: <20180207151854.GB10945@tassilo.jf.intel.com>
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> 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