Re: [RFC PATCH 0/3] iommu: Add range flush operation

[Russell King - ARM Linux]

On Tue, Sep 29, 2015 at 03:20:38PM +0100, Robin Murphy wrote:
> A single callback doesn't really generalise well enough: If we wanted to
> implement this in the ARM SMMU drivers to optimise the unmap() case [ask
> Will how long he spends waiting for a software model to tear down an entire
> VFIO domain invalidating one page at a time ;)], then we'd either regress
> performance in the map() case with an unnecessary TLB flush, or have to do a
> table walk in every flush() call to infer what actually needs doing.

And this is the problem of frameworks.  They get in the way of doing
things efficiently.

Fine, we have the DMA ops, and that calls a map_sg() method.  What we
then need is to have a series of standardised library functions which
can be called to perform various actions.

Consider this: an IOMMU driver gets the raw scatterlist which the
driver passed.  The IOMMU driver walks the scatterlist, creating the
IOMMU side mapping, and writing the device DMA addresses and DMA lengths
to the scatterlist, possibly coalescing some of the entries.  It
remembers the number of scatterlist entries that the DMA operation now
requires.  The IOMMU code can setup whatever mappings it wants using
whatever sizes it wants to satisfy the requested scatterlist.

It then goes on to call the arch backend with the original scatterlist,
asking it to _only_ deal with the CPU coherency for the mapping.  The
arch code walks the scatterlist again, this time dealing with the CPU
coherency part.

Finally, the IOMMU code returns the number of DMA scatterlist entries.

When it comes to tearing it down, it's a similar operation to the above,
except reversing those actions.

The only issue with this approach is that it opens up some of the cache
handling to the entire kernel, and that will be _too_ big a target for
idiotic driver writers to think they have permission to directly use
those interfaces.  To solve this, I'd love to be able to have the linker
link together certain objects in the kernel build, and then convert some
global symbols to be local symbols, thus denying access to functions that
driver authors have no business what so ever touching.

> Personally I think it would be nicest to have two separate callbacks, e.g.
> .map_sync/.unmap_sync, but at the very least some kind of additional
> 'direction' kind of parameter would be necessary.

No, not more callbacks - that's the framework thinking, not the library
thinking.

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