Re: [PATCH v3 0/3] virtio DMA API core stuff

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2015-11-19 at 13:59 -0800, Andy Lutomirski wrote:
> 
> >
> > So thinking hard about it, I don't see any real drawbacks to making this
> > conditional on a new feature bit, that Xen can then set..
> 
> Can you elaborate?  If I run QEMU, hosting Xen, hosting Linux, and the
> virtio device is provided by QEMU, then how does Xen set the bit?
> Similarly, how would Xen set the bit for a real physical device?

Right. This is *not* a fundamental characteristic of the device. This
is all about how your *particular* hypervisor (in the set of turtles-
all-the-way-down) happened to expose the thing to you.

This is why it lives in the DMAR table, in the Intel world, which
*tells* you which devices are behind which IOMMU (and which are not).
And why I keep repeating myself that it has nothing to do with the
actual device or the virtio drivers.

I understand that POWER and other platforms don't currently have a
clean way to indicate that certain device don't have translation. And I
understand that we may end up with a *quirk* which ensures that the DMA
API does the right thing (i.e. nothing) in certain cases.

But we should *NOT* be involving the virtio device drivers in that
quirk, in any way. And putting a feature bit in the virtio device
itself doesn't seem at all sane either.

Bear in mind that qemu-system-x86_64 currently has the *same* problem
with assigned physical devices. It's claiming they're translated, and
they're not.

-- 
dwmw2


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