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

[Yan Zhao]

On Thu, Jun 06, 2024 at 08:55:03AM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 06, 2024 at 10:48:10AM +0800, Yan Zhao wrote:
> > On Sat, Jun 01, 2024 at 04:46:14PM -0300, Jason Gunthorpe wrote:
> > > On Mon, May 27, 2024 at 11:37:34PM -0700, Christoph Hellwig wrote:
> > > > On Tue, May 21, 2024 at 01:00:16PM -0300, Jason Gunthorpe wrote:
> > > > > > > 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?
> > > > 
> > > > Just have a unmap version that also takes a list of PFNs that you'd
> > > > need for non-coherent mappings?
> > > 
> > > VFIO has never supported that so nothing like that exists yet.. This
> > > is sort of the first steps to some very basic support for a
> > > non-coherent cache flush in a limited case of a VM that can do its own
> > > cache flushing through kvm.
> > > 
> > > The pfn list is needed for unpin_user_pages() and it has an ugly
> > > design where vfio/iommufd read back the pfns seperately from unmap,
> > > and they both do it differently without a common range list
> > > datastructure here.
> > > 
> > > So, we'd need to build some new unmap function that returns a pfn list
> > > that it internally fetches via the read ops. Then it can do the read,
> > > unmap, flush iotlb, flush cache in core code.
> > Would the core code flush CPU caches by providing page physical address?
> 
> Physical address is all we will have in the core code..
> 
> > If yes, do you think it's still necessary to export arch_flush_cache_phys()
> > (as what's implemented in this patch)?
> 
> Christoph is asking not to export it, that would mean relying on the
> iommu core to be non-modulare and putting the arch calls there with a
> more restricted exported API - ie based on unmap.

Got it. Thanks for explanation!
> 
> > > I've been working towards this very slowly as I want to push this
> > > stuff down into the io page table walk and remove the significant
> > > inefficiency, so it is not throw away work, but it is certainly some
> > > notable amount of work to do.
> > Will VFIO also be switched to this new unmap interface? Do we need to care
> > about backporting?
> 
> I don't know :)
>  
> > And is it possible for VFIO alone to implement in the current proposed way
> > in this series as the first step for easier backport?
> 
> I think this series is the best option we have right now, but make the
> EXPORT a NS export to try to discourage abuse of it while we continue
> working
Will do. Thanks!