Re: [RFC 0/4] Virtio uses DMA API for all devices

From: Benjamin Herrenschmidt
Date: Tue Aug 07 2018 - 16:34:26 EST

On Tue, 2018-08-07 at 06:55 -0700, Christoph Hellwig wrote:
> On Tue, Aug 07, 2018 at 04:42:44PM +1000, Benjamin Herrenschmidt wrote:
> > Note that I can make it so that the same DMA ops (basically standard
> > swiotlb ops without arch hacks) work for both "direct virtio" and
> > "normal PCI" devices.
> >
> > The trick is simply in the arch to setup the iommu to map the swiotlb
> > bounce buffer pool 1:1 in the iommu, so the iommu essentially can be
> > ignored without affecting the physical addresses.
> >
> > If I do that, *all* I need is a way, from the guest itself (again, the
> > other side dosn't know anything about it), to force virtio to use the
> > DMA ops as if there was an iommu, that is, use whatever dma ops were
> > setup by the platform for the pci device.
> In that case just setting VIRTIO_F_IOMMU_PLATFORM in the flags should
> do the work (even if that isn't strictly what the current definition
> of the flag actually means). On the qemu side you'll need to make
> sure you have a way to set VIRTIO_F_IOMMU_PLATFORM without emulating
> an iommu, but with code to take dma offsets into account if your
> plaform has any (various power plaforms seem to have them, not sure
> if it affects your config).

Something like that yes. I prefer a slightly different way, see below,
any but in both cases, it should alleviate your concerns since it means
there would be no particular mucking around with DMA ops at all, virtio
would just use whatever "normal" ops we establish for all PCI devices
on that platform, which will be standard ones.

(swiotlb ones today and the new "integrates" ones you're cooking

As for the flag itself, while we could set it from qemu when we get
notified that the guest is going secure, both Michael and I think it's
rather gross, it requires qemu to go iterate all virtio devices and
"poke" something into them.

It also means qemu will need some other internal nasty flag that says
"set that bit but don't do iommu".

It's nicer if we have a way in the guest virtio driver to do something
along the lines of

if ((flags & VIRTIO_F_IOMMU_PLATFORM) || arch_virtio_wants_dma_ops())

Which would have the same effect and means the issue is entirely
contained in the guest.