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:

quoted

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.

quoted

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