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

Will Deacon
2019-02-11T17:33:04Z

Hi Arnd,

On Mon, Feb 11, 2019 at 06:11:48PM +0100, Arnd Bergmann wrote:
> 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.

So I've just sent an RFC (you're on cc) that attempts to rewrite this part
of memory-barriers.txt to reflect reality. Hopefully that can act as a
starting point for discussion if we decide we want to change the documented
behaviour and/or implementation.

Will