Re: [PATCH v4 0/6] virtio core DMA API conversion

[Michael S. Tsirkin]

On Tue, Nov 10, 2015 at 09:37:54PM +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2015-11-09 at 21:35 -0800, Andy Lutomirski wrote:
> > 
> > We could do it the other way around: on powerpc, if a PCI device is in
> > that range and doesn't have the "bypass" property at all, then it's
> > assumed to bypass the IOMMU.  This means that everything that
> > currently works continues working.  If someone builds a physical
> > virtio device or uses another system in PCIe target mode speaking
> > virtio, then it won't work until they upgrade their firmware to set
> > bypass=0.  Meanwhile everyone using hypothetical new QEMU also gets
> > bypass=0 and no ambiguity.
> >
> > vfio will presumably notice the bypass and correctly refuse to map any
> > current virtio devices.
> > 
> > Would that work?
> 
> That would be extremely strange from a platform perspective. Any device
> in that vendor/device range would bypass the iommu unless some new
> property "actually-works-like-a-real-pci-device" happens to exist in
> the device-tree, which we would then need to define somewhere and
> handle accross at least 3 different platforms who get their device-tree 
> from widly different places.

Then we are back to virtio driver telling DMA core
whether it wants a 1:1 mapping in the iommu?
If that's acceptable to others, I don't think that's too bad.


> Also if tomorrow I create a PCI device that implements virtio-net and
> put it in a machine running IBM proprietary firmware (or Apple's or
> Sun's), it won't have that property...
> 
> This is not hypothetical. People are using virtio to do point-to-point
> communication between machines via PCIe today.
> 
> Cheers,
> Ben.

But not virtio-pci I think - that's broken for that usecase since we use
weaker barriers than required for real IO, as these have measureable
overhead.  We could have a feature "is a real PCI device",
that's completely reasonable.

-- 
MST
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