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

From: Linus Torvalds
Date: Mon Jan 07 2019 - 12:44:06 EST

On Mon, Jan 7, 2019 at 1:55 AM David Laight <David.Laight@xxxxxxxxxx> 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.