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

[David Laight <David.Laight@ACULAB.COM>]

From: Linus Torvalds
> Sent: 07 January 2019 17:44
> 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.

Actually they tend to handle it on a lot of systems.
(But I don't do it.)
Probably most of those where vm_iomap_memory() (to map IO memory
space directly into user pages) works.

It might be 'interesting' to build an amd64 kernel where all the IO
memory addresses (eg returned by pci_iomap()) are offset by a
large constant so direct accesses all fault and all the readl()
macros (etc) add it back in.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)