Re: [PATCH v3 0/7] Allow setting caching mode in arch_add_memory() for P2PDMA

Jason Gunthorpe
2020-02-27T17:43:14Z

On Thu, Feb 27, 2020 at 10:21:50AM -0700, Logan Gunthorpe wrote:
>
>
> On 2020-02-27 10:17 a.m., Jason Gunthorpe wrote:
> >> Instead of this, this series proposes a change to arch_add_memory()
> >> to take the pgprot required by the mapping which allows us to
> >> explicitly set pagetable entries for P2PDMA memory to WC.
> >
> > Is there a particular reason why WC was selected here? I thought for
> > the p2pdma cases there was no kernel user that touched the memory?
>
> Yes, that's correct. I choose WC here because the existing users are
> registering memory blocks without side effects which fit the WC
> semantics well.

Hm, AFAIK WC memory is not compatible with the spinlocks/mutexs/etc in
Linux, so while it is true the memory has no side effects, there would
be surprising concurrency risks if anything in the kernel tried to
write to it.

Not compatible means the locks don't contain stores to WC memory the
way you would expect. AFAIK on many CPUs extra barriers are required
to keep WC stores ordered, the same way ARM already has extra barriers
to keep UC stores ordered with locking..

The spinlocks are defined to contain UC stores though.

If there is no actual need today for WC I would suggest using UC as
the default.

Jason