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

Logan Gunthorpe
2020-02-27T17:55:12Z

On 2020-02-27 10:43 a.m., Jason Gunthorpe wrote:
> 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.

Ok, that sounds sensible. I'll do that in the next revision.

Thanks,

Logan