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SubjectRe: [PATCH RFC] fixup! virtio: convert to use DMA api
On Mon, Apr 18, 2016 at 02:29:33PM -0400, David Woodhouse wrote:
> 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?

Depends on how SoC is designed I guess. At the moment specifically QEMU
runs everything in a single memory space so an IOMMU table lookup does
not offer any extra protection. That's not a must, one could come
up with modular hypervisor designs - it's just what we have ATM.

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

That's a policy decision though.

> On the whole, there are *plenty* of reasons why we might want to have a
> passthrough mapping on a per-device basis,

That's true. And driver security also might differ, for example maybe I
trust a distro-supplied driver more than an out of tree one. Or maybe I
trust a distro-supplied userspace driver more than a closed-source one.
And maybe I trust devices from same vendor as my chip more than a 3rd
party one. So one can generalize this even further, think about device
and driver security/trust level as an integer and platform protection as an

If platform IOMMU offers you extra protection over trusting the device
(trust < protection) it improves you security to use platform to limit
the device. If trust >= protection it just adds overhead without
increasing the security.

> and I really struggle to
> find justification for having this 'hint' in a virtio-specific way.

It's a way. No system seems to expose this information in a more generic
way at the moment, and it's portable. Would you like to push for some
kind of standartization of such a hint? I would be interested
to hear about that.

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

I guess you are right in that we should split this part out.
What I wanted is really the combination
PASSTHROUGH && !PLATFORM so that we can say "ok we don't
need to guess, this device actually bypasses the IOMMU".

And I thought it's a nice idea to use PASSTHROUGH && PLATFORM
as a hint since it seemed to be unused.
But maybe the best thing to do for now is to say
- hosts should not set PASSTHROUGH && PLATFORM
- guests should ignore PASSTHROUGH if PLATFORM is set

and then we can come back to this optimization idea later
if it's appropriate.

So yes I think we need the two bits but no we don't need to
mix the hint discussion in here.

> > 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
> bugs.

I guess we differ here. I care about fixing bugs and not breaking
working setups but I see little value in working around
existing bugs if they can be fixed at their source.

Building a generic mechanism to report which devices bypass the IOMMU
isn't trivial because there's no simple generic way to address
an arbitrary device from hypervisor. For example, DMAR tables
commonly use bus numbers for that but these are guest (bios) assigned.
So if we used bus numbers we'd have to ask bios to build a custom
ACPI table and stick bus numbers there.

> 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.

With iommu enabled? Point is, I don't really care about that.
At this point only a very small number of devices work with this
IOMMU at all. I expect that we'll fix assigned devices very soon.

> 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.

Absolutely but that's a separate discussion.

> 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.

I don't see how in that legacy kernels bypassed the DMA API.
To me it looks like we either use physical addresses that they give us
or they don't work at all (at least without iommu=pt),
since the VT-D spec says:
DMA requests processed through root-entries with present field
Clear result in translation-fault.

So I suspect the IOMMU_PLATFORM flag would have to stay off
by default for a while.

> 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.

Right. That would also be an issue.

> > 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.

Right. However, the way timing happens to work, we are out of time to
fix it in 2.6 and we are highly likely to have the proper VFIO fix in
2.7. So I'm not sure there's space for a short term fix.

> > >
> > > 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.

Well it does matter for people doing it :)

> And if there isn't, the
> whole discussion is moot anyway.

Point was that we can always build a paravirt interface
if it does not exist, but it's easier to maintain
if it's minimal, being as close to emulating hardware
as we can.

> --
> dwmw2

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