Re: [PATCH] iommu/dma: Handle MSI mappings separately

On 30/07/2019 07:28, Christoph Hellwig wrote:

On Mon, Jul 29, 2019 at 04:32:38PM +0100, Robin Murphy wrote:

quoted

MSI pages must always be mapped into a device's *current* domain, which
*might* be the default DMA domain, but might instead be a VFIO domain
with its own MSI cookie. This subtlety got accidentally lost in the
streamlining of __iommu_dma_map(), but rather than reintroduce more
complexity and/or special-casing, it turns out neater to just split this
path out entirely.

Since iommu_dma_get_msi_page() already duplicates much of what
__iommu_dma_map() does, it can easily just make the allocation and
mapping calls directly as well. That way we can further streamline the
helper back to exclusively operating on DMA domains.

Fixes: b61d271e59d7 ("iommu/dma: Move domain lookup into __iommu_dma_{map,unmap}")
Reported-by: Shameer Kolothum <redacted>
Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Hmm.  I remember proposing this patch and you didn't like it because
we could also have msis for a !IOMMU_DMA_IOVA_COOKIE cookie type.
Or did we talk past each other?

Do you have a pointer? That sparks the vaguest of memories, but I can't 
seem to turn anything up in my inbox. If that was my objection, though, 
it sounds like your patch was probably trying to go a step or two 
further than this one.

Note that if this change turns out to be valid we should also
clean up the iommu_dma_free_iova() side.

We're not touching the iommu_dma_{alloc,free}_iova() path here; those 
are designed to cope with both types of cookie, and I think that's a 
reasonable abstraction to keep. This is just getting rid of the 
asymmetry - and now bug - caused by trying to keep the MSI page flow 
going through a special case in __iommu_dma_map() despite that having 
evolved into a more specific DMA domain fastpath (there's no 
corresponding unmap special case since MSI mappings just persist and get 
recycled until the domain is destroyed).

Robin.

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