Re: [PATCH 3/5] x86/mm: Introduce and export interface arch_clean_nonsnoop_dma()

Jason Gunthorpe
2024-05-21T16:00:25Z

On Tue, May 21, 2024 at 12:49:39PM -0300, Jason Gunthorpe wrote:
> On Mon, May 20, 2024 at 07:07:10AM -0700, Christoph Hellwig wrote:
> > On Tue, May 07, 2024 at 02:20:44PM +0800, Yan Zhao wrote:
> > > Introduce and export interface arch_clean_nonsnoop_dma() to flush CPU
> > > caches for memory involved in non-coherent DMAs (DMAs that lack CPU cache
> > > snooping).
> >
> > Err, no. There should really be no exported cache manipulation macros,
> > as drivers are almost guaranteed to get this wrong. I've added
> > Russell to the Cc list who has been extremtly vocal about this at least
> > for arm.
>
> We could possibly move this under some IOMMU core API (ie flush and
> map, unmap and flush), the iommu APIs are non-modular so this could
> avoid the exported symbol.

Though this would be pretty difficult for unmap as we don't have the
pfns in the core code to flush. I don't think we have alot of good
options but to make iommufd & VFIO handle this directly as they have
the list of pages to flush on the unmap side. Use a namespace?

Jason