Re: Virtio in Xen on Arm (based on IOREQ concept)

[Oleksandr <olekstysh@xxxxxxxxx>]

On 20.07.20 23:38, Stefano Stabellini wrote:

Hello Stefano

> On Fri, 17 Jul 2020, Oleksandr wrote:
> > > > *A few word about solution:*
> > > > As it was mentioned at [1], in order to implement virtio-mmio Xen on Arm
> > > Any plans for virtio-pci? Arm seems to be moving to the PCI bus, and
> > > it would be very interesting from a x86 PoV, as I don't think
> > > virtio-mmio is something that you can easily use on x86 (or even use
> > > at all).
> > Being honest I didn't consider virtio-pci so far. Julien's PoC (we are based
> > on) provides support for the virtio-mmio transport
> >
> > which is enough to start working around VirtIO and is not as complex as
> > virtio-pci. But it doesn't mean there is no way for virtio-pci in Xen.
> >
> > I think, this could be added in next steps. But the nearest target is
> > virtio-mmio approach (of course if the community agrees on that).
> Hi Julien, Oleksandr,
>
> Aside from complexity and easy-of-development, are there any other
> architectural reasons for using virtio-mmio?
>
> I am not asking because I intend to suggest to do something different
> (virtio-mmio is fine as far as I can tell.) I am asking because recently
> there was a virtio-pci/virtio-mmio discussion recently in Linaro and I
> would like to understand if there are any implications from a Xen point
> of view that I don't yet know.
Unfortunately, I can't say anything regarding virtio-pci/MSI. Could the 
virtio-pci work in virtual environment without PCI support (various 
embedded platforms)?
It feels to me that both transports (easy and lightweight virtio-mmio 
and complex and powerfull virtio-pci) will have their consumer demand 
and worth being implemented in Xen.

> For instance, what's your take on notifications with virtio-mmio? How
> are they modelled today? Are they good enough or do we need MSIs?
Notifications are sent from device (backend) to the driver (frontend) 
using interrupts. Additional DM function was introduced for that purpose 
xendevicemodel_set_irq_level() which results in vgic_inject_irq() call.
Currently, if device wants to notify a driver it should trigger the 
interrupt by calling that function twice (high level at first, then low 
level).

--
Regards,

Oleksandr Tyshchenko