Re: [PATCH v4 1/1] Introduce MGMT Admin commands

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 4/5/2022 1:29 AM, Michael S. Tsirkin wrote:
> On Wed, Mar 02, 2022 at 06:02:37PM +0200, Max Gurtovoy wrote:
> > +It is beyond the scope of the virtio specification to define necessary synchronization in system software to ensure that a virtio PCI VF device
> > +interrupt configuration modification is reflected in the PCI device. However, it is expected that any modern system software implementing virtio
> > +drivers and PCI subsystem will ensure that any changes occurring in the VF interrupt configuration is either updated in the PCI VF device or
> > +such configuration fails.
> I am no longer sure this assertion holds. For example, how would the PF
> driver ensure that e.g. VFIO is not bound to a VF for passthrough to
> a VM and is not configuring the MSI-X configuration of the VF?
> I don't really see a way to do that cleanly.
>
> Would you care to post a proof of concept or even a pseudo-code patch?

I don't think we a POC for now.

We have an example of MSI-X configuration in mlx5 driver: 
https://lore.kernel.org/linux-pci/20210314124256.70253-1-leon@kernel.org/

The infrastructure in Linux already exist.

> It appears even harder to support in an (admittedly, uncommon,
> but apparently available due to virtio subsystem not necessarily
> matching the PF boundary) case where the admin queue is
> in a VF and so the admin driver is running within guest.

I'm not sure I follow.

If the VF will support AQ it doesn't mean it will have all the optional 
functionality we're adding to the PF.

> I thus have been thinking of an alternate approach, where the # of MSIX
> vectors does not change, but the # of VQs does.  Since guests do not
> currently request more vectors than VQs, this will address the
> requirement in a cleaner way that guests should be able to universally
> support.  Synchronization then can be achieved by failing the command if
> any status bits (or just VIRTIO_CONFIG_S_DRIVER?  did not think too
> deeply about the difference ...) in the affected VFs are set indicating
> an attached driver. A new feature bit might be required for this and
> maybe a new field indicating the actual # of vectors.

We discussed about the VQs settings in the past. It is more challenging 
thing to do since each device type has it's own VQ types and configurations.

MSI-X is a common feature that we can apply on each virtio device.

>