RE: [virtio-dev] Re: [PATCH 2/5] Introduce VIRTIO_F_ADMIN_VQ_INDIRECT_DESC/VIRTIO_F_ADMIN_VQ_IN_ORDER

[Parav Pandit <parav@nvidia.com>]

> From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> Behalf Of Michael S. Tsirkin
> Sent: Wednesday, January 19, 2022 12:51 PM

> > > Um. Interesting. This means that the msix capability of the VF changes?
> > Yes.
> > > Is that in fact spec compliant? Could some OSes cache the value of
> > > the capability even if the device is not in active use? E.g. I can
> > > see how this might happen in order to map the MSIX tables even before
> loading the driver.
> > >
> > PCI subsystem can catch the value before the device driver can load.
> > Generally a device support intx/msix or intx/msi. So PCI subsystem is not
> aware what will be used by its upper layer device drivers.
> > So it usually differs such initialization to a later stage until it is actually used.
> >
> > Whichever OS driver which implements msix configuration, will have to
> either not cache it or flush+ rebuild the cache.
> 
> Seems to contradict what the spec says (below).
No it doesn't. spec covers that dependency is on system software policy, system sw, driver sw.
So this system that contains policy, sw, driver sw will implement virtio extension.

In above sentence of "whichever OS driver", it covers all sw components involved in this functionality.
> 
> > > The spec says:
> > > 	Depending upon system software policy, system software, device
> > > driver software, or each at
> > > 	different times or environments may configure a functionâs MSI-X
> > > capability and table
> > > 	structures with suitable vectors.
> > >
> > > So MSIX canfiguation might not be up to the driver.
> > >
> > > We actually ask driver to read back any vector assigned to a VQ so
> > > it's possible to fail vector assignment. Maybe that's better.
> > >
> > Virtio driver should not incur any additional complexity in re-reading vector
> etc.
> 
> I think it does this already.
When does it re-read?
I do not follow your point of "ask driver to read back any vector". When do you want to do this?

> 
> > All the msix config should happen much before drivers gets loaded for the VF.
> > It is PCI layer of the HV to provide a stable device to virtio device driver which
> is not undergoing msix table changes, when virtio device driver is operating on
> it.
> 
> 
> Problem is in the guest though. I'm not sure we can rely on this part being part
> of the driver and not part of the OS.
It is part of the system software that consist of virtio driver, pci subsystem and user interface.
I do not follow your comment about "problem is in guest though".
Can you please explain?
VF is simply not available to the guest, when HV has not given it. And when its given, HV doesnât modify the msix in some random manner.