Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs

Benjamin Herrenschmidt
2020-09-02T23:08:58Z

On Thu, 2020-09-03 at 09:08 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2020-09-02 at 18:21 +0100, Catalin Marinas wrote:
> > > I understand we do want weak ordering for prefetchable BAR
> > > mappings
> > > but my worry is that by exposing the resources as WC to user
> > > space
> > > we are giving user space the impression that those mappings
> > > mirror
> > > x86 WC mappings behaviour that is not true on ARM64.
> >
> > Would Device_GRE be close to the x86 WC better? It won't allow
> > unaligned
> > accesses and that can be problematic for the user. OTOH, it doesn't
> > speculate reads, so it's safer from the hardware perspective.
>
> Its accepted generally that prefetchable BARs can allow speculative
> accesses, write combining, re-ordering even etc... and it's also
> commonly be the target of unaligned accesses.
>
> In reality, in PCI land, it really means "no side effects".

Correction: No *read* side effects.

Cheers,
Ben.