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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
Date: Tue, 6 Feb 2018 15:54:02 -0800	[thread overview]
Message-ID: <20180206235402.GN30338@tassilo.jf.intel.com> (raw)
In-Reply-To: <CA+55aFyXfMhUNoU9WveiMyKq32LftUGzF-BAP7hxEQOfwBTJrA@mail.gmail.com>

> The reason for that complexity is purely the system call fastpath case
> that no longer exists, I think.
> 
> Am I missing something?

Yes merging the macros should be fine without fast path. 

But for push, on older CPUs (older AMD, most Atoms, really old Intel big core) 
sub+mov is a lot faster than push because push has additional dependencies
causing pipeline bubbles. So you would make these cases slower if you
use PUSH.

That is no different between fast path and slow path.

-Andi

PS it was never fully clear to me why we removed the fast path. After all it 
could still be useful on the future CPUs with Spectre hardware fixes.

  parent reply	other threads:[~2018-02-06 23:54 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-06  1:18 [PATCH v3 0/3] x86/entry: Clear registers to sanitize speculative usages Dan Williams
2018-02-06  1:18 ` [PATCH v3 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels Dan Williams
2018-02-06 11:52   ` [tip:x86/pti] x86/entry/64: Clear extra registers beyond syscall arguments, to reduce speculation attack surface tip-bot for Dan Williams
2018-02-06  1:18 ` [PATCH v3 2/3] x86/entry: Clear registers for 64bit exceptions/interrupts Dan Williams
2018-02-06  9:04   ` Dominik Brodowski
2018-02-06 10:48     ` Ingo Molnar
2018-02-06  9:17   ` Dominik Brodowski
2018-02-06 10:51     ` Ingo Molnar
2018-02-06 10:57       ` Dominik Brodowski
2018-02-06 21:25       ` [PATCH tip-pti 1/2] x86/entry: remove SAVE_C_REGS_EXCEPT_* macros Dominik Brodowski
2018-02-06 22:56         ` Linus Torvalds
2018-02-06 21:32       ` [PATCH tip-pti 2/2] x86/entry: interleave XOR register clearing with PUSH/MOV instructions Dominik Brodowski
2018-02-06 22:30         ` Dan Williams
2018-02-06 22:48         ` Linus Torvalds
2018-02-06 23:05           ` Andy Lutomirski
2018-02-06 23:54           ` Andi Kleen [this message]
2018-02-07  1:30             ` Linus Torvalds
2018-02-07 15:18               ` Andi Kleen
2018-02-07 17:05                 ` Linus Torvalds
2018-02-07 17:37                   ` Linus Torvalds
2018-02-06 12:00   ` [tip:x86/pti] x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface tip-bot for Dan Williams
2018-02-06  1:18 ` [PATCH v3 3/3] x86/entry: Clear registers for compat syscalls Dan Williams
2018-02-06  7:26   ` Ingo Molnar
2018-02-06  7:53     ` Dan Williams
2018-02-06 12:00   ` [tip:x86/pti] x86/entry/64/compat: Clear registers for compat syscalls, to reduce speculation attack surface tip-bot for Dan Williams

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