Hi Tomasz,

On Mon, Mar 23, 2015 at 05:38:45PM +0900, Tomasz Figa wrote:

While unmapping, the driver zaps all iovas belonging to the mapping,
so the page tables not used by any mapping won't be cached. Now when
the driver creates a mapping it might end up occupying several page
tables. However, since the mapping area is virtually contiguous, only
the first and last page table can be shared with different mappings.
This means that only first and last iovas can be already cached. In
fact, we could detect if first and last page tables are shared and do
not zap at all, but this wouldn't really optimize too much. Why
invalidating one iova is enough to invalidate the whole page table is
unclear to me as well, but it seems to be the correct way on this
hardware.

As for the race, it's also kind of explained by the above. The already
running hardware can trigger page table look-ups in the IOMMU and so
caching of the page table between our zapping and updating its
contents. With this patch zapping is performed after updating the page
table so the race is gone.

Okay, this makes sense. Can you add this information to the patch
changelog and resend please?

Thanks,

	Joerg