Re: [PATCH V2 vfio 2/9] virtio-pci: Introduce admin virtqueue

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Parav Pandit <parav@nvidia.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Maor Gottlieb <maorg@nvidia.com>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	Jason Gunthorpe <jgg@nvidia.com>, Jiri Pirko <jiri@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>
Subject: Re: [PATCH V2 vfio 2/9] virtio-pci: Introduce admin virtqueue
Date: Mon, 30 Oct 2023 19:31:38 -0400	[thread overview]
Message-ID: <20231030193110-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <PH0PR12MB548197CD7A10D5A89B7213CDDCA1A@PH0PR12MB5481.namprd12.prod.outlook.com>

On Mon, Oct 30, 2023 at 06:10:06PM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Monday, October 30, 2023 9:29 PM
> > On Mon, Oct 30, 2023 at 03:51:40PM +0000, Parav Pandit wrote:
> > >
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Monday, October 30, 2023 1:53 AM
> > > >
> > > > On Sun, Oct 29, 2023 at 05:59:45PM +0200, Yishai Hadas wrote:
> > > > > From: Feng Liu <feliu@nvidia.com>
> > > > >
> > > > > Introduce support for the admin virtqueue. By negotiating
> > > > > VIRTIO_F_ADMIN_VQ feature, driver detects capability and creates
> > > > > one administration virtqueue. Administration virtqueue
> > > > > implementation in virtio pci generic layer, enables multiple types
> > > > > of upper layer drivers such as vfio, net, blk to utilize it.
> > > > >
> > > > > Signed-off-by: Feng Liu <feliu@nvidia.com>
> > > > > Reviewed-by: Parav Pandit <parav@nvidia.com>
> > > > > Reviewed-by: Jiri Pirko <jiri@nvidia.com>
> > > > > Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> > > > > ---
> > > > >  drivers/virtio/virtio.c                | 37 ++++++++++++++--
> > > > >  drivers/virtio/virtio_pci_common.c     |  3 ++
> > > > >  drivers/virtio/virtio_pci_common.h     | 15 ++++++-
> > > > >  drivers/virtio/virtio_pci_modern.c     | 61 +++++++++++++++++++++++++-
> > > > >  drivers/virtio/virtio_pci_modern_dev.c | 18 ++++++++
> > > > >  include/linux/virtio_config.h          |  4 ++
> > > > >  include/linux/virtio_pci_modern.h      |  5 +++
> > > > >  7 files changed, 137 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > > index
> > > > > 3893dc29eb26..f4080692b351 100644
> > > > > --- a/drivers/virtio/virtio.c
> > > > > +++ b/drivers/virtio/virtio.c
> > > > > @@ -302,9 +302,15 @@ static int virtio_dev_probe(struct device *_d)
> > > > >  	if (err)
> > > > >  		goto err;
> > > > >
> > > > > +	if (dev->config->create_avq) {
> > > > > +		err = dev->config->create_avq(dev);
> > > > > +		if (err)
> > > > > +			goto err;
> > > > > +	}
> > > > > +
> > > > >  	err = drv->probe(dev);
> > > > >  	if (err)
> > > > > -		goto err;
> > > > > +		goto err_probe;
> > > > >
> > > > >  	/* If probe didn't do it, mark device DRIVER_OK ourselves. */
> > > > >  	if (!(dev->config->get_status(dev) & VIRTIO_CONFIG_S_DRIVER_OK))
> > > >
> > > > Hmm I am not all that happy that we are just creating avq unconditionally.
> > > > Can't we do it on demand to avoid wasting resources if no one uses it?
> > > >
> > > Virtio queues must be enabled before driver_ok as we discussed in
> > F_DYNAMIC bit exercise.
> > > So creating AQ when first legacy command is invoked, would be too late.
> > 
> > Well we didn't release the spec with AQ so I am pretty sure there are no devices
> > using the feature. Do we want to already make an exception for AQ and allow
> > creating AQs after DRIVER_OK even without F_DYNAMIC?
> > 
> No. it would abuse the init time config registers for the dynamic things like this.
> For flow filters and others there is need for dynamic q creation with multiple physical address anyway.

That seems like a completely unrelated issue.

> So creating virtqueues dynamically using a generic scheme is desired with new feature bit.

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