Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

[Simona Vetter]

On Thu, Jan 09, 2025 at 01:56:02AM +0800, Xu Yilun wrote:
> > > > 5) iommufd and kvm are both using CPU addresses without DMA. No
> > > > exporter mapping is possible
> > > 
> > > We have customers using both KVM and XEN with DMA-buf, so I can clearly
> > > confirm that this isn't true.
> > 
> > Today they are mmaping the dma-buf into a VMA and then using KVM's
> > follow_pfn() flow to extract the CPU pfn from the PTE. Any mmapable
> > dma-buf must have a CPU PFN.
> 
> Yes, the final target for KVM is still the CPU PFN, just with the help
> of CPU mapping table.
> 
> I also found the xen gntdev-dmabuf is calculating pfn from mapped
> sgt.

See the comment, it's ok because it's a fake device with fake iommu and
the xen code has special knowledge to peek behind the curtain.
-Sima
 
> From Christion's point, I assume only sgl->dma_address should be
> used by importers but in fact not. More importers are 'abusing' sg dma
> helpers.
> 
> That said there are existing needs for importers to know more about the
> real buffer resource, for mapping, or even more than mapping,
> e.g. dmabuf_imp_grant_foreign_access()
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch