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Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api

From: David Woodhouse
Subject: Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api
Date: Mon, 18 Apr 2016 11:51:41 -0400

On Mon, 2016-04-18 at 18:30 +0300, Michael S. Tsirkin wrote:
> > Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that
> > its own operating system's IOMMU code is expected to be broken, and
> > that the virtio driver should eschew the DMA API?
> No - it tells guest that e.g. the ACPI tables (or whatever the
> equivalent is) do not match reality with respect to this device
> since IOMMU is ignored by hypervisor.
> Hypervisor has no idea what does guest IOMMU code do - hopefully
> it is not actually broken.

OK, that makes sense — thanks.

So where the platform *does* have a way to coherently tell the guest
that some devices are behind and IOMMU and some aren't, we should never
perhaps temporarily on x86 until we *do* fix the DMAR tables to tell
the truth; qv.)

This should *only* be a crutch for platforms which cannot properly
convey that information from the hypervisor to the guest. It should be
clearly documented "thou shalt not use this unless you've first
attempted to fix the broken platform to get it right for itself".

And if we look at it as such... does it make more sense for this to be
a more *generic* qemu←→guest interface? That way the software hacks can
live in the OS IOMMU code where they belong, and prevent assignment to
nested guests for example. And can cover cases like assigned PCI
devices in existing qemu/x86 which need the same treatment.

Put another way: if we're going to add code to the guest OS to look at
this information, why can't we add that code in the guest's IOMMU
support instead, to look at an out-of-band qemu-specific "ignore IOMMU
for these devices" list instead?

> The status quo is that that the IOMMU might well be bypassed
> and then you need to program physical addresses into the device,
> but maybe not. If DMA API does not give you physical addresses, you
> need to bypass it, but hypervisor does not know or care.

Right. The status quo is that qemu doesn't provide correct information
about IOMMU topology to guests, and they have to have heuristics to
work out whether to eschew the IOMMU for a given device or not. This is
true for virtio and assigned PCI devices alike.

Furthermore, some platforms don't *have* a standard way for qemu to
'tell the truth' to the guests, and that's where the real fun comes in.
But still, I'd like to see a generic solution for that lack instead of
a virtio-specific hack.


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