Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

[David Woodhouse <dwmw2@infradead.org>]

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> > On some systems, including Xen and any system with a physical device
> > that speaks virtio behind a physical IOMMU, we must use the DMA API
> > for virtio DMA to work at all.
> > 
> > Add a feature bit to detect that: VIRTIO_F_IOMMU_PLATFORM.
> > 
> > If not there, we preserve historic behavior and bypass the DMA
> > API unless within Xen guest. This is actually required for
> > systems, including SPARC and PPC64, where virtio-pci devices are
> > enumerated as though they are behind an IOMMU, but the virtio host
> > ignores the IOMMU, so we must either pretend that the IOMMU isn't
> > there or somehow map everything as the identity.
> > 
> > Re: non-virtio devices.
> > 
> > It turns out that on old QEMU hosts, only emulated devices which were
> > part of QEMU use the IOMMU.  Should we want to bypass the IOMMU for such
> > devices *only*, it would be rather easy to detect them by looking at
> > subsystem vendor and device ID. Thus, no new interfaces are required
> > except for virtio which always uses the same subsystem vendor and device ID.

Apologies for dropping this thread; I've been travelling.

But seriously, NO!

I understand why you want to see this as a virtio-specific issue, but
it isn't. And we don't *want* it to be.

In the guest, drivers SHALL use the DMA API. And the DMA API SHALL do
the right thing for each device according to its needs.

So any information passed from qemu to the guest should be directed at
the platform IOMMU code (or handled by qemu-detection quirks in the
guest, if we must).

It is *not* acceptable for the virtio drivers in the guest to just
eschew the DMA API completely, triggered by some device-specific flag.

The qemu implementation is, of course, monolithic. In qemu the fact
that virtio doesn't get translated by the emulated IOMMU *is* actually
down to code in the virtio implementation. I get that.

But then again, it's not just virtio. *Any* device which we emulate for
the guest could have that same issue, and appear as untranslated. (And
assigned PCI devices currently do).

Let's think about the parallel with a system-on-chip. Let's say we have
a peripheral which got included, but which was wired up such that it
bypasses the IOMMU and gets to do direct physical DMA. Is that a
feature of that specific peripheral? Do we hack its drivers to make the
distinction between this incarnation, and a normal discrete version of
the same device? No! It's a feature of the *system* and needs to be
conveyed to the OS IOMMU code to do the right thing. Not to the driver.

In my opinion, adding the VIRTIO_F_IOMMU_PLATFORM feature bit is
absolutely the wrong thing to do.

What we *should* do is a patchset in the guest which both fixes virtio
drivers to *always* use the DMA API, and fixes the DMA API to DTRT at
the same time — by detecting qemu and installing no-op DMA ops for the
appropriate devices, perhaps.

Then we can look at giving qemu a way to properly indicate which
devices it actually does DMA mapping for, so we can remove those
heuristic assumptions.

But that flag does *not* live in the virtio host←→guest ABI.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation


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