Re: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

Arnd Bergmann
2019-02-11T17:11:48Z

On Mon, Feb 11, 2019 at 4:30 PM Will Deacon <will.deacon@arm.com> wrote:

> Given the lack of Intel response here, I went away to do some digging.
> As evidenced by the commit message, there is certainly an understanding
> amongst some developers that inX/outX() are strongly ordered on x86 and
> this was re-enforced by Linus in March last year:
>
> https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg131212.html
>
> It was this information on which I based my patch. The Intel SDM is not
> quite as assertive in its claims.
>
> However, it has also occurred to me that this patch is actually missing
> the point. memory-barriers.txt should be documenting the *Linux* memory
> model, not the x86 one, and so the port accessors should be defined to
> have the same ordering semantics as the MMIO accessors. If this wasn't
> the case, then macros such as ioreadX() and iowriteX() would be unusable
> in portable driver code.

My interpretation of the ioreadX() and iowriteX() semantics is that they
only guarantee readl()/writel() barrier semantics, even though they
may in fact provide stronger barriers for PIO on architectures that use
CONFIG_GENERIC_IOMAP (which falls back to inX()/outX()).

> The inX/outX implementation in asm-generic would
> also be bogus, despite being widely used.

They likely are. The asm-generic files tend to provide a generic
abstraction as much as that is possible, but without having access
to the architecture specific semantics, they raditionally don't know
what should be done here. We now have __io_pbw()/__io_paw()/
__io_pbr()/__io_par() to let architectures get it right, but that is
a fairly recent addition, so nothing other than riscv defines them
today.
To make things worse, a lot of machines are unable to provide
__io_paw(), e.g. when all bus writes are posted.

Arnd