Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in non-coherent domains

[Yan Zhao]

On Wed, May 22, 2024 at 02:01:50PM -0300, Jason Gunthorpe wrote:
> On Wed, May 22, 2024 at 02:29:19PM +0800, Yan Zhao wrote:
> > > > If you want to minimize flushes then you can't store flush
> > > > minimization information in the pages because it isn't global to the
> > > > pages and will not be accurate enough.
> > > > 
> > > > > > If pfn_reader_fill_span() does batch_from_domain() and
> > > > > > the source domain's storage_domain is non-coherent then you can skip
> > > > > > the flush. This is not pedantically perfect in skipping all flushes, but
> > > > > > in practice it is probably good enough.
> > > > 
> > > > > We don't know whether the source storage_domain is non-coherent since
> > > > > area->storage_domain is of "struct iommu_domain".
> > > >  
> > > > > Do you want to add a flag in "area", e.g. area->storage_domain_is_noncoherent,
> > > > > and set this flag along side setting storage_domain?
> > > > 
> > > > Sure, that could work.
> > > When the storage_domain is set in iopt_area_fill_domains(),
> > >     "area->storage_domain = xa_load(&area->iopt->domains, 0);"
> > > is there a convenient way to know the storage_domain is non-coherent?
> > Also asking for when storage_domain is switching to an arbitrary remaining domain
> > in iopt_unfill_domain().
> > 
> > And in iopt_area_unfill_domains(), after iopt_area_unmap_domain_range()
> > of a non-coherent domain which is not the storage domain, how can we know that
> > the domain is non-coherent?
> 
> Yes, it would have to keep track of hwpts in more case unfortunately
> :(
Current iopt keeps an xarray of domains. Could I modify the xarray to store
hwpt instead?