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
