netdev - Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

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Message-ID: <CA+55aFydu4+MZ=-5L7wDr2LvNH6Dbmi4qLnfHAAOrHKZCb5wdA@mail.gmail.com>
Date:   Wed, 21 Mar 2018 18:26:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Ganesh GR <ganeshgr@...lsio.com>,
        Nirranjan Kirubaharan <nirranjan@...lsio.com>,
        Indranil Choudhury <indranil@...lsio.com>
Subject: Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

On Tue, Mar 20, 2018 at 7:42 AM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> Instead of framing this as an enhanced version of the read/write ops
> why not look at replacing or extending something like the
> memcpy_fromio or memcpy_toio operations?

Yes, doing something like "memcpy_fromio_avx()" is much more
palatable, in that it works like the crypto functions do - if you do
big chunks, the "kernel_fpu_begin/end()" isn't nearly the issue it can
be otherwise.

Note that we definitely have seen hardware that *depends* on the
regular memcpy_fromio()" not doing big reads. I don't know how
hardware people screw it up, but it's clearly possible.

So it really needs to be an explicitly named function that basically a
driver can use to say "my hardware really likes big aligned accesses"
and explicitly ask for some AVX version if possible.

                    Linus