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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 14:29:33 -0400

On Mon, 2016-04-18 at 19:27 +0300, Michael S. Tsirkin wrote:
> I balk at adding more hacks to a broken system. My goals are
> merely to
> - make things work correctly with an IOMMU and new guests,
>   so people can use userspace drivers with virtio devices
> - prevent security risks when guest kernel mistakenly thinks
>   it's protected by an IOMMU, but in fact isn't
> - avoid breaking any working configurations

AFAICT the VIRTIO_F_IOMMU_PASSTHROUGH thing seems orthogonal to this.
That's just an optimisation, for telling an OS "you don't really need
to bother with the IOMMU, even though you it works".

There are two main reasons why an operating system might want to use
the IOMMU via the DMA API for native drivers: 
 - To protect against driver bugs triggering rogue DMA.
 - To protect against hardware (or firmware) bugs.

With virtio, the first reason still exists. But the second is moot
because the device is part of the hypervisor and if the hypervisor is
untrustworthy then you're screwed anyway... but then again, in SoC
devices you could replace 'hypervisor' with 'chip' and the same is
true, isn't it? Is there *really* anything virtio-specific here?

Sure, I want my *external* network device on a PCIe card with software-
loadable firmware to be behind an IOMMU because I don't trust it as far
as I can throw it. But for on-SoC devices surely the situation is
*just* the same as devices provided by a hypervisor?

And some people want that external network device to use passthrough
anyway, for performance reasons.

On the whole, there are *plenty* of reasons why we might want to have a
passthrough mapping on a per-device basis, and I really struggle to
find justification for having this 'hint' in a virtio-specific way.

And it's complicating the discussion of the *actual* fix we're looking

> Looking at guest code, it looks like virtio was always
> bypassing the IOMMU even if configured, but no other
> guest driver did.
> This makes me think the problem where guest drivers
> ignore the IOMMU is virtio specific
> and so a virtio specific solution seems cleaner.
> The problem for assigned devices is IMHO different: they bypass
> the guest IOMMU too but no guest driver knows about this,
> so guests do not work. Seems cleaner to fix QEMU to make
> existing guests work.

I certainly agree that it's better to fix QEMU. Whether devices are
behind an IOMMU or not, the DMAR tables we expose to a guest should
tell the truth.

Part of the issue here is virtio-specific; part isn't.

Basically, we have a conjunction of two separate bugs which happened to
work (for virtio) — the IOMMU support in QEMU wasn't working for virtio
(and assigned) devices even though it theoretically *should* have been,
and the virtio drivers weren't using the DMA API as they theoretically
should have been.

So there were corner cases like assigned PCI devices, and real hardware
implementations of virtio stuff (and perhaps virtio devices being
assigned to nested guests) which didn't work. But for the *common* use
case, one bug cancelled out the other.

Now we want to fix both bugs, and of course that involves carefully
coordinating both fixes.

I *like* your idea of a flag from the hypervisor which essentially says
"trust me, I'm telling the truth now".

But don't think that wants to be virtio-specific, because we actually
want it to cover *all* the corner cases, not just the common case which
*happened* to work before due to the alignment of the two previous

An updated guest OS can look for this flag (in its generic IOMMU code)
and can apply a heuristic of its own to work out which devices *aren't*
behind the IOMMU, if the flag isn't present. And it can get that right
even for assigned devices, so that new kernels can run happily even on
today's QEMU instances. And the virtio driver in new kernels should
just use the DMA API and expect it to work. Just as the various drivers
for assigned PCI devices do.

The other interesting case for compatibility is old kernels running in
a new QEMU. And for that case, things are likely to break if you
suddenly start putting the virtio devices behind an IOMMU. There's
nothing you can do on ARM and Power to stop that breakage, since they
don't *have* a way to tell legacy guests that certain devices aren't
translated. So I suspect you probably can't enable virtio-behind-IOMMU
in QEMU *ever* for those platforms as the default behaviour.

For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
the truth, and even legacy kernels ought to cope with that.
FSVO 'ought to' where I suspect some of them will actually crash with a
NULL pointer dereference if there's no "catch-all" DMAR unit in the
tables, which puts it back into the same camp as ARM and Power.

> True but I think we should fix QEMU to shadow IOMMU
> page tables for assigned devices. This seems rather
> possible with VT-D, and there are patches already on list.
> It looks like this will fix all legacy guests which is
> much nicer than what you suggest which will only help new guests.

Yes, we should do that. And in the short term we should at *least* fix
the DMAR tables to tell the truth.

> > 
> > 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.
> But the issue is not just these holes.  E.g. with VT-D it is only easy
> to emulate because there's a "caching mode" hook. It is fundamentally
> paravirtualization.  So a completely generic solution would be a
> paravirtualized IOMMU interface, replacing VT-D for VMs. It might be
> justified if many platforms have hard to emulate interfaces.

Hm, I'm not sure I understand the point here.

Either there is a way for the hypervisor to expose an IOMMU to a guest
(be it full hardware virt, or paravirt). Or there isn't.

If there is, it doesn't matter *how* it's done. And if there isn't, the
whole discussion is moot anyway.


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