Re: [RFC 0/4] Virtio uses DMA API for all devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 30, 2018 at 04:18:02AM -0700, Christoph Hellwig wrote:
> On Mon, Jul 30, 2018 at 01:28:03PM +0300, Michael S. Tsirkin wrote:
> > Let me reply to the "crappy" part first:
> > So virtio devices can run on another CPU or on a PCI bus. Configuration
> > can happen over mupltiple transports.  There is a discovery protocol to
> > figure out where it is. It has some warts but any real system has warts.
> > 
> > So IMHO virtio running on another CPU isn't "legacy virtual crappy
> > virtio". virtio devices that actually sit on a PCI bus aren't "sane"
> > simply because the DMA is more convoluted on some architectures.
> 
> All of what you said would be true if virtio didn't claim to be
> a PCI device.  

There's nothing virtio claims to be.  It's a PV device that uses PCI for
its configuration.  Configuration is enumerated on the virtual PCI bus.
That part of the interface is emulated PCI. Data path is through a
PV device enumerated on the virtio bus.

> Once it claims to be a PCI device and we also see
> real hardware written to the interface I stand to all what I said
> above.

Real hardware would reuse parts of the interface but by necessity it
needs to behave slightly differently on some platforms.  However for
some platforms (such as x86) a PV virtio driver will by luck work with a
PCI device backend without changes. As these platforms and drivers are
widely deployed, some people will deploy hardware like that.  Should be
a non issue as by definition it's transparent to guests.

> > With this out of my system:
> > I agree these approaches are hacky. I think it is generally better to
> > have virtio feature negotiation tell you whether device runs on a CPU or
> > not rather than rely on platform specific ways for this. To this end
> > there was a recent proposal to rename VIRTIO_F_IO_BARRIER to
> > VIRTIO_F_REAL_DEVICE.  It got stuck since "real" sounds vague to people,
> > e.g.  what if it's a VF - is that real or not? But I can see something
> > like e.g. VIRTIO_F_PLATFORM_DMA gaining support.
> > 
> > We would then rename virtio_has_iommu_quirk to virtio_has_dma_quirk
> > and test VIRTIO_F_PLATFORM_DMA in addition to the IOMMU thing.
> 
> I don't really care about the exact naming, and indeed a device that
> sets the flag doesn't have to be a 'real' device - it just has to act
> like one.  I explained all the issues that this means (at least relating
> to DMA) in one of the previous threads.

I believe you refer to this:
https://lkml.org/lkml/2018/6/7/15
that was a very helpful list outlining the problems we need to solve,
thanks a lot for that!

> The important bit is that we can specify exact behavior for both
> devices that sets the "I'm real!" flag and that ones that don't exactly
> in the spec.

I would very much like that, yes.

> And that very much excludes arch-specific (or
> Xen-specific) overrides.

We already committed to a xen specific hack but generally I prefer
devices that describe how they work instead of platforms magically
guessing, yes.

However the question people raise is that DMA API is already full of
arch-specific tricks the likes of which are outlined in your post linked
above. How is this one much worse?

-- 
MST