Re: [PATCH v5 2/3] virtio_pci: Use the DMA API for virtqueues when possible

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2014-09-17 at 09:07 -0700, Andy Lutomirski wrote:
> It shouldn't.  That being said, at some point this problem will need
> solving on PPC, and this patch doesn't help much, other than adding
> the virtio_ring piece.
> 
> I'd really like to see the generic or arch IOMMU code handle this so
> that the PPC special case in the virtio-pci driver can go away.
> 
> Another reason that this kind of special casing doesn't belong in
> virtio-pci: if anyone ever binds vfio to a virtio-pci device, if the
> DMA API doesn't work right, then the kernel will go boom or be
> compromised.

Well, that's also for these reasons that I think your patch shouldn't go
through those virtio_* variants that test that "use_dma_api" flag or
whatever you called it last time I looked :-)

You should just unconditionally call the DMA API and we should fixup the
special cases by overriding the hooks.

I can help you do the override hack for PPC for now, until we can do
something saner accross all architectures.

The main problem is s390 but I had a quick chat with Utz Bacher earlier
this week and he seemed to think having s390 use the DMA API shouldn't
be a big deal, they can also hookup some transparent set of ops if
necessary.

Cheers,
Ben.