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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 7 Jan 2019 09:43:41 -0800
Message-ID: <CAHk-=wg02_--Xbf9HQw0g5-0k_uY+ksbxfBHK45BKDnPjz9OXw@mail.gmail.com>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes
To:     David Laight <David.Laight@aculab.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        "dvlasenk@redhat.com" <dvlasenk@redhat.com>,
        Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "bp@alien8.de" <bp@alien8.de>,
        Peter Anvin <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "brgerst@gmail.com" <brgerst@gmail.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        "pabeni@redhat.com" <pabeni@redhat.com>
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On Mon, Jan 7, 2019 at 1:55 AM David Laight <David.Laight@aculab.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