RE: [PATCH v4 05/17] iommufd: Separate kernel-managed HWPT alloc/destroy/abort functions

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Friday, October 13, 2023 10:06 PM
> 
> On Fri, Oct 13, 2023 at 07:13:34AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Friday, October 13, 2023 3:10 AM
> > >
> > > Also, we probably should feed enforce_cache_coherency through the
> > > alloc_hwpt uapi and not try to autodetect it..
> > >
> >
> > In the past we had a long discussion about this with the conclusion
> > that user opt is the ideal model but it's fine to stay with autodetect
> > and existing vfio/kvm contract for coherency detection until we
> > see a real demand for user opt.
> >
> > Is there anything new which changes your mind to have user opt now?
> 
> I guess, I was just looking at the complexity it brings to keep that
> working.
> 

vfio_file_enforced_coherent() currently just looks at device_iommu_capable()
and can be called before attach, assuming an autodetect model. 

moving away from autodetect would need a new contract which I hesitate
to bother with at this point.