Re: [Xen-devel] OASIS Virtual I/O Device (VIRTIO) TC

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Sorry for late reply but I was on vaction. Slowly recovering.

On Thu, Aug 15, 2013 at 04:09:04PM +0100, Wei Liu wrote:
> On Thu, Aug 15, 2013 at 03:58:46PM +0100, Ian Campbell wrote:
> > On Thu, 2013-08-15 at 15:24 +0100, Wei Liu wrote:
> > > On Thu, Aug 15, 2013 at 10:16:41AM -0400, Konrad Rzeszutek Wilk wrote:
> > > > On Thu, Aug 15, 2013 at 02:37:28PM +0100, Wei Liu wrote:
> > > > > On Tue, Aug 13, 2013 at 08:41:28PM +0200, Daniel Kiper wrote:
> > > > > > Hi,
> > > > > >
> > > > > > I was appointed by Xen Advisory Board to OASIS Virtual I/O Device
> > > > > > (VIRTIO) TC as a memeber. I will oversee its work from Xen point
> > > > > > of view, however, deliverables will be as much as possible
> > > > > > "virtualization platform" agnostic.
> > > > > >
> > > > > > According to [1]:
> > > > > >
> > > > > > The goal of the OASIS Virtual I/O Device (VIRTIO) TC is to simplify
> > > > > > virtual devices, making them more extensible and more recognizable.
> > > > > >
> > > > > > [...]
> > > > > >
> > > > > > The TC intends to define formal specifications for virtual device 
> > > > > > buses
> > > > > > (including PCI) for a variety of devices, including network devices.
> > > > > > Specification development will be based upon the "Virtio PCI Card
> > > > > > Specification" [2] v0.9.5, seeking solutions that support 
> > > > > > portability,
> > > > > > simplicity, least-surprise for driver authors, extensibility, and
> > > > > > performance. The specification will also document existing
> > > > > > implementations and practice.
> > > > > >
> > > > >
> > > > > How is Xen PV driver fit in this? Xen PV doesn't have virtual PCI bus.
> > > >
> > > >
> > >
> > > My limited knowledge of Virtio is pretty dated now, that's why I raised
> > > that question. ;-)
> > >
> > > > One could implement an PCI over XenBus I suppose. Or just write an
> > > > XenBus PCI driver that would do all the neccessary things to respond
> > > > to the proper commands.
> > > > >
> > >
> > > Back in the date my impression was that XenBus is asynchronuous while
> > > virtual PCI is synchronuous (i.e. trap-process-return), they are not
> > > quite compatible.
> > >
> > > Furtuer more using XenBus is mainly used for configuration, while Virtio
> > > over PCI supports both configuration and sending notifications etc.
> > >
> > > I'm not sure XenBus PCI driver would be a good idea...
> >
> > I thought this part of virtio was pluggable and that PCI was one option
> > (although for a long time the only one). On ARM you can also use
>
> Yes this is true.
>
> However the statement (?) in Daniel's first email (based upon the
> "Virtio PCI Card Specification") gave me the impression that they
> planned to support virtual PCI only...

Right, VIRTIO TC started its work on the base of "Virtio PCI Card 
Specification".
However, it was stated immediately that PCI specification should be excluded
from main specification and should form a separate attachment. It means that
VIRTIO would not require PCI bus in the future. Additionally, current document
contains specification for MMIO which looks like good alternative for PCI.

> > virt_mmio instead these days. A shared ring for cfg + evtchn model for
> > notification doesn't seem too far fetched to me.
> >
> > A far bigger problem IMHO with virtio is that it basically discards the
> > Xen security model. I don't know how hard it will be to retrofit gnttab
> > based access control into the virtio protocols. A lot would be my
> > guess...

I am going to convince VIRTIO TC that new spec should be Xen friendly too.

> Indeed, seems there is lots of work to do...

By the way, Wei, Konrad told me that you worked on VRITIO implementation
for Xen and you have hands-on-experience with it. Could you tell me what
issues you met during your work on VRITIO support for Xen environment?

Daniel

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