Re: [PATCH v5 07/17] dma-mapping: Implement link/unlink ranges API

[Leon Romanovsky <leon@kernel.org>]

On Wed, Jan 15, 2025 at 07:26:28AM +0100, Christoph Hellwig wrote:
> On Tue, Jan 14, 2025 at 08:50:35PM +0000, Robin Murphy wrote:

<...>

> >> +	if (dev_use_swiotlb(dev, size, dir) && iova_offset(iovad, phys | size))
> >
> > Again, why are we supporting non-granule-aligned mappings in the middle of 
> > a range when the documentation explicitly says not to?
> 
> It's not trying to support that, but checking that this is guaranteed
> to be the last one is harder than handling it like this.  If you have
> a suggestion for better checks that would be very welcome.
> 
> >> +		if (!dev_is_dma_coherent(dev) &&
> >> +		    !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> >> +			arch_sync_dma_for_cpu(phys, len, dir);
> >
> > Hmm, how do attrs even work for a bulk unlink/destroy when the individual 
> > mappings could have been linked with different values?
> 
> They shouldn't.  Just like randomly mixing flags doesn't work for the
> existing APIs.
> 
> > (So no, irrespective of how conceptually horrid it is, clearly it's not 
> > even functionally viable to open-code abuse of DMA_ATTR_SKIP_CPU_SYNC in 
> > callers to attempt to work around P2P mappings...)
> 
> What do you mean with "work around"?  I guess Leon added it to the hmm
> code based on previous feedback, but I still don't think any of our P2P
> infrastructure works reliably with non-coherent devices as
> iommu_dma_map_sg gets this wrong.  So despite the earlier comments I
> suspect this should stick to the state of the art even if that is broken.

Right, I was asked to set DMA_ATTR_SKIP_CPU_SYNC for PCI_P2PDMA_MAP_THRU_HOST_BRIDGE case.
...
  752         case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
  753                 attrs |= DMA_ATTR_SKIP_CPU_SYNC;
  754                 pfns[idx] |= HMM_PFN_P2PDMA;
  755                 break;

At this stage, we didn't change DMA/IOMMU previous behaviour and if it
was broken for certain flows, it stays to be broken after this series
too.

Thanks