Re: [PATCH rfc] vfio-pci: Allow write combining

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Aug 01, 2024 at 10:52:18AM -0600, Alex Williamson wrote:
> > > vfio_region_info.flags in not currently tested for input therefore this
> > > proposal could lead to unexpected behavior for a caller that doesn't
> > > currently zero this field.  It's intended as an output-only field.  
> > 
> > Perhaps a REGION_INFO2 then?
> > 
> > I still think per-request is better than a global flag
> 
> I don't understand why we'd need a REGION_INFO2, we already have
> support for defining new regions.

It is not a new region, it is a modified mmap behavior for an existing
region.

> We'd populate these new regions only for BARs that support prefetch and
> mmap 

That's not the point, prefetch has nothing to do with write combining.

Every BAR can be mapped writecombining, it is up to the VFIO userspace
to understand if it can use it or not. The only use case for this
feature would be something like in DPDK.

VM side write combining is already handled by KVM allowing the VM to
upgrade the page attributes to WC from NC.

Doubling all the region indexes just for WC does not seem like a good
idea to me...

Jason