linux-kernel - Re: Question Regarding ERMS memcpy

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Message-ID: <20170304224341.zfp4fl37ypt57amg@pd.tnic>
Date:   Sat, 4 Mar 2017 23:43:41 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question Regarding ERMS memcpy

On Sat, Mar 04, 2017 at 01:08:15PM -0700, Logan Gunthorpe wrote:
> So my question is: how do I find out what version of memcpy my actual
> machine is using and fix it if it is wrong?

You can boot with "debug-alternative" and look for those strings where
it says "feat:"

[    0.261386] apply_alternatives: feat: 3*32+18, old: (ffffffff8101e3c4, len: 3), repl: (ffffffff81de4e7a, len: 3), pad: 3
					 ^^^^^^^

[    0.264003] ffffffff8101e3c4: old_insn: 0f 1f 00
[    0.265235] ffffffff81de4e7a: rpl_insn: 0f ae e8
[    0.266469] ffffffff8101e3c4: final_insn: 0f ae e8

if it says 9*32+9 there, then it really does patch in the ERMS variant.

If it says 3*32+16 and it patches a NOP, i.e., something like this:

[    2.022665] apply_alternatives: feat: 3*32+16, old: (ffffffff812dbfe0, len: 5), repl: (ffffffff81de5336, len: 0), pad: 3
[    2.024003] ffffffff812dbfe0: old_insn: eb 0e 90 90 90
[    2.025332] ffffffff812dbfe0: final_insn: 0f 1f 44 00 00
					     ^^^^ NOP

then, you're using the rep; movsq variant.

Also, do

make <path-to-file>.s

of the file where it does memcpy_fromio() and attach it here. I'd like to see
what the compiler generates.

HTH.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--