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

From: Michael S. Tsirkin
Date: Thu Nov 12 2015 - 06:10:10 EST

On Wed, Nov 11, 2015 at 11:30:27PM +0100, David Woodhouse wrote:
> On Wed, 2015-11-11 at 07:56 -0800, Andy Lutomirski wrote:
> >
> > Can you flesh out this trick?
> >
> > On x86 IIUC the IOMMU more-or-less defaults to passthrough. If the
> > kernel wants, it can switch it to a non-passthrough mode. My patches
> > cause the virtio driver to do exactly this, except that the host
> > implementation doesn't actually exist yet, so the patches will instead
> > have no particular effect.
> At some level, yes â we're compatible with a 1982 IBM PC and thus the
> IOMMU is entirely disabled at boot until the kernel turns it on â
> except in TXT mode where we abandon that compatibility.
> But no, the virtio driver has *nothing* to do with switching the device
> out of passthrough mode. It is either in passthrough mode, or it isn't.
> If the VMM *doesn't* expose an IOMMU to the guest, obviously the
> devices are in passthrough mode. If the guest kernel doesn't have IOMMU
> support enabled, then obviously the devices are in passthrough mode.
> And if the ACPI tables exposed to the guest kernel *tell* it that the
> virtio devices are not actually behind the IOMMU (which qemu gets
> wrong), then it'll be in passthrough mode.
> 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.

Intel's iommu can be programmed to still
do a kind of passthrough (1:1) mapping, it's
just a matter of doing this for virtio devices
when not using VFIO.

> 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 â 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.

> > On powerpc and sparc, we *already* screwed up. The host already tells
> > the guest that there's an IOMMU and that it's *enabled* because those
> > platforms don't have selective IOMMU coverage the way that x86 does.
> > So we need to work around it.
> No, we need it on x86 too because once we fix the virtio device driver
> bug and make it start using the DMA API, then we start to trip up on
> the qemu bug where it lies about which devices are covered by the
> Of course, we still have that same qemu bug w.r.t. assigned devices,
> which it *also* claims are behind its IOMMU when they're not...

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.

> > I think that, if we want fancy virt-friendly IOMMU stuff like you're
> > talking about, then the right thing to do is to create a virtio bus
> > instead of pretending to be PCI. That bus could have a virtio IOMMU
> > and its own cross-platform enumeration mechanism for devices on the
> > bus, and everything would be peachy.
> That doesn't really help very much for the x86 case where the problem
> is compatibility with *existing* (arguably broken) qemu
> implementations.
> 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.

> > I still don't understand what trick. If we want virtio devices to be
> > assignable, then they should be translated through the IOMMU, and the
> > DMA API is the right interface for that.
> The DMA API is the right interface *regardless* of whether there's
> actual translation to be done. The device driver itself should not be
> involved in any way with that decision.

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.

> When you want to access MMIO, you use ioremap() and writel() instead of
> doing random crap for yourself. When you want DMA, you use the DMA API
> to get a bus address for your device *even* if you expect there to be
> no IOMMU and you expect it to precisely match the physical address. No
> excuses.

No problem, but the fact remains that virtio does need
per-device control over whether it's passthrough or not.

Forget the bugs, that's not the issue - the issue is
that it's sometimes part of hypervisor and
sometimes isn't.

We just can't say it's always not a part of hypervisor so you always
want maximum protection - that drops performance by to the floor.

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.

> --
> dwmw2

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