linux-kernel - Re: [PATCH] x86: only use ERMS for user copies for larger sizes

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Date:   Mon, 7 Jan 2019 09:43:41 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        "dvlasenk@...hat.com" <dvlasenk@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
        Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        "brgerst@...il.com" <brgerst@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On Mon, Jan 7, 2019 at 1:55 AM David Laight <David.Laight@...lab.com> wrote:
>
> I needed to open-code one part because it wants to do copy_to_user()
> from a PCIe address buffer (which has to work).

It will never work for memcpy_fromio(). Any driver that thinks it will
copy from io space to user space absolutely *has* to do it by hand. No
questions, and no exceptions. Some loop like

   for (..)
      put_user(readl(iomem++), uaddr++);

because neither copy_to_user() nor memcpy_fromio() will *ever* handle
that correctly.

They might randomly happen to work on x86, but absolutely nowhere else.

               Linus