Re: [kvm-unit-tests PATCH v9 13/12] io: Unify IO accessors across architectures

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/07/2016 22:39, Alexander Gordeev wrote:
> > > We could drop the wrappers, and require unit tests to use barriers
> > > explicitly, when needed. I'm OK with that, because I think I mostly
> > > forget that we have them in the wrappers and use them explicitly
> > > anyway...  I'll need to double check all the uses though.
> > 
> > According to my findings read*/write* io accessors are used on
> > ioremap-ed areas in lib/arm/io.c and lib/virtio-mmio.c at the moment.
> > 
> > Even if read*/write* wrappers are stripped of SMP barriers,
> > removing compiler barriers from them would be a bad idea AFAICT.

I agree (though ACCESS_ONCE-style volatile access will do).

> > So if we need compiler barriers in read*/write* then why not
> > just leave SMP barriers there?

Fair enough, I'm applying the rest of the series.

Paolo

> Andrew, Paolo,
> 
> I am paraphrasing myself. So read*/write* are used extensively
> by lib/virtio-mmio.c to access memory mapped registers. AFAICT
> we do want serialize accesses to devices and need barriers for
> that. The same applies to lib/pci-testdev.c when/if it accepted.
>