Re: [Qemu-devel] [PATCH V2 RFC] fixup! virtio: convert to use DMA api

[Michael S. Tsirkin]

On Wed, Apr 27, 2016 at 01:18:21PM +0100, David Woodhouse wrote:
> 
> > > On some systems, including Xen and any system with a physical device
> > > that speaks virtio behind a physical IOMMU, we must use the DMA API
> > > for virtio DMA to work at all.
> > > 
> > > Add a feature bit to detect that: VIRTIO_F_IOMMU_PLATFORM.
> > > 
> > > If not there, we preserve historic behavior and bypass the DMA
> > > API unless within Xen guest. This is actually required for
> > > systems, including SPARC and PPC64, where virtio-pci devices are
> > > enumerated as though they are behind an IOMMU, but the virtio host
> > > ignores the IOMMU, so we must either pretend that the IOMMU isn't
> > > there or somehow map everything as the identity.
> > > 
> > > Re: non-virtio devices.
> > > 
> > > It turns out that on old QEMU hosts, only emulated devices which were
> > > part of QEMU use the IOMMU.  Should we want to bypass the IOMMU for such
> > > devices *only*, it would be rather easy to detect them by looking at
> > > subsystem vendor and device ID. Thus, no new interfaces are required
> > > except for virtio which always uses the same subsystem vendor and device 
> > > ID.
> 
> Apologies for dropping this thread; I've been travelling.
> 
> But seriously, NO!
> 
> I understand why you want to see this as a virtio-specific issue, but
> it isn't. And we don't *want* it to be.
> 
> In the guest, drivers SHALL use the DMA API. And the DMA API SHALL do
> the right thing for each device according to its needs.
> 
> So any information passed from qemu to the guest should be directed at
> the platform IOMMU code (or handled by qemu-detection quirks in the
> guest, if we must).
> 
> It is *not* acceptable for the virtio drivers in the guest to just
> eschew the DMA API completely, triggered by some device-specific flag.
> 
> The qemu implementation is, of course, monolithic. In qemu the fact
> that virtio doesn't get translated by the emulated IOMMU *is* actually
> down to code in the virtio implementation. I get that.
> 
> But then again, it's not just virtio. *Any* device which we emulate for
> the guest could have that same issue, and appear as untranslated. (And
> assigned PCI devices currently do).
> 
> Let's think about the parallel with a system-on-chip. Let's say we have
> a peripheral which got included, but which was wired up such that it
> bypasses the IOMMU and gets to do direct physical DMA. Is that a
> feature of that specific peripheral? Do we hack its drivers to make the
> distinction between this incarnation, and a normal discrete version of
> the same device? No! It's a feature of the *system*

One correction: it's a feature of the device in the system.
There could be a mix of devices bypassing and not
bypassing the IOMMU.

> and needs to be
> conveyed to the OS IOMMU code to do the right thing. Not to the driver.
> 
> In my opinion, adding the VIRTIO_F_IOMMU_PLATFORM feature bit is
> absolutely the wrong thing to do.
> 
> What we *should* do is a patchset in the guest which both fixes virtio
> drivers to *always* use the DMA API, and fixes the DMA API to DTRT at
> the same time — by detecting qemu and installing no-op DMA ops for the
> appropriate devices, perhaps.

Sounds good. And a way to detect appropriate devices could
be by looking at the feature flag, perhaps?


> Then we can look at giving qemu a way to properly indicate which
> devices it actually does DMA mapping for, so we can remove those
> heuristic assumptions.
> 
> But that flag does *not* live in the virtio host←→guest ABI.
> 
> -- 
> David Woodhouse                            Open Source Technology Centre
> address@hidden                              Intel Corporation
>