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

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2015-11-12 at 13:09 +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 11, 2015 at 11:30:27PM +0100, David Woodhouse wrote:
> > 
> > If the IOMMU is exposed, and enabled, and telling the guest kernel that
> > it *does* cover the virtio devices, then those virtio devices will
> > *not* be in passthrough mode.
> 
> This we need to fix. Because in most configurations if you are
> using kernel drivers, then you don't want IOMMU with virtio,
> but if you are using VFIO then you do.

This is *absolutely* not specific to virtio. There are *plenty* of
other users (especially networking) where we only really care about the
existence of the IOMMU for VFIO purposes and assigning devices to
guests, and we are willing to dispense with the protection that it
offers for native in-kernel drivers. For that, boot with iommu=pt.

There is no way, currently, to enable the passthrough mode on a per-
device basis. Although it has been discussed right here, very recently.

Let's not conflate those issues.

> > You choosing to use the DMA API in the virtio device drivers instead of
> > being buggy, has nothing to do with whether it's actually in
> > passthrough mode or not. Whether it's in passthrough mode or not, using
> > the DMA API is technically the right thing to do — because it should
> > either *do* the translation, or return a 1:1 mapped IOVA, as
> > appropriate.
> 
> Right but first we need to actually make DMA API do the right thing
> at least on x86,ppc and arm.

It already does the right thing on x86, modulo BIOS bugs (including the
qemu ACPI table but that you said you're not too worried about).

> I'm not worried about qemu bugs that much.  I am interested in being
> able to use both VFIO and kernel drivers with virtio devices with good
> performance and without tweaking kernel parameters.

OK, then you are interested in the semi-orthogonal discussion about
DMA_ATTR_IOMMU_BYPASS. Either way, device drivers SHALL use the DMA
API.


> > Having said that, if this were real hardware I'd just be blacklisting
> > it and saying "Another BIOS with broken DMAR tables --> IOMMU
> > completely disabled". So perhaps we should just do that.
> > 
> Yes, once there is new QEMU where virtio is covered by the IOMMU,
> that would be one way to address existing QEMU bugs. 

No, that's not required. All that's required is to fix the currently-
broken ACPI table so that it *admits* that the virtio devices aren't
covered by the IOMMU. And I've never waited for a fix to be available
before, before blacklisting *other* broken firmwares...

The only reason I'm holding off for now is because ARM and PPC also
need a quirk for their platform code to realise that certain devices
actually *aren't* covered by the IOMMU, and I might be able to just use
the same thing and still enable the IOMMU in the offending qemu
versions.

Although as noted, it would need to cover assigned devices as well as
virtio — qemu currently lies to us and tells us that the emulated IOMMU
in the guest does cover *those* too.

> With virt, each device can have different priveledges:
> some are part of hypervisor so with a kernel driver
> trying to get protection from them using an IOMMU which is also
> part of hypervisor makes no sense
>  - but when using a
> userspace driver then getting protection from the userspace
> driver does make sense. Others are real devices so
> getting protection from them makes some sense.
> 
> Which is which? It's easiest for the device driver itself to
> gain that knowledge. Please note this is *not* the same
> question as whether a specific device is covered by an IOMMU.

OK. How does your device driver know whether the virtio PCI device it's
talking to is actually implemented by the hypervisor, or whether it's
one of the real PCI implementations that apparently exist?

> Linux doesn't seem to support that usecase at the moment, if this is a
> generic problem then we need to teach Linux to solve it, but if virtio
> is unique in this requirement, then we should just keep doing virtio
> specific things to solve it.

It is a generic problem. There is a discussion elsewhere about how (or
indeed whether) to solve it. It absolutely isn't virtio-specific, and
we absolutely shouldn't be doing virtio-specific things to solve it.

Nothing excuses just eschewing the correct DMA API. That's just broken,
and only ever worked in conjunction with *other* bugs elsewhere in the
platform.


-- 
dwmw2


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