Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 27 Jul 2022 11:45:43 -0400

On Wed, Jul 27, 2022 at 05:50:59PM +0800, Jason Wang wrote:
> On Wed, Jul 27, 2022 at 5:03 PM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Wed, Jul 27, 2022 at 02:54:13PM +0800, Jason Wang wrote:
> > > On Wed, Jul 27, 2022 at 2:01 PM Michael S. Tsirkin <[email-masked]> wrote:
> > > >
> > > > On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
> > > > >
> > > > > > From: Zhu, Lingshan <[email-masked]>
> > > > > > Sent: Tuesday, July 26, 2022 10:53 PM
> > > > > >
> > > > > > On 7/27/2022 10:17 AM, Parav Pandit wrote:
> > > > > > >> From: Zhu, Lingshan <[email-masked]>
> > > > > > >> Sent: Tuesday, July 26, 2022 10:15 PM
> > > > > > >>
> > > > > > >> On 7/26/2022 11:56 PM, Parav Pandit wrote:
> > > > > > >>>> From: Zhu, Lingshan <[email-masked]>
> > > > > > >>>> Sent: Tuesday, July 12, 2022 11:46 PM
> > > > > > >>>>> When the user space which invokes netlink commands, detects that
> > > > > > >> _MQ
> > > > > > >>>> is not supported, hence it takes max_queue_pair = 1 by itself.
> > > > > > >>>> I think the kernel module have all necessary information and it is
> > > > > > >>>> the only one which have precise information of a device, so it
> > > > > > >>>> should answer precisely than let the user space guess. The kernel
> > > > > > >>>> module should be reliable than stay silent, leave the question to
> > > > > > >>>> the user space
> > > > > > >> tool.
> > > > > > >>> Kernel is reliable. It doesn’t expose a config space field if the
> > > > > > >>> field doesn’t
> > > > > > >> exist regardless of field should have default or no default.
> > > > > > >> so when you know it is one queue pair, you should answer one, not try
> > > > > > >> to guess.
> > > > > > >>> User space should not guess either. User space gets to see if _MQ
> > > > > > >> present/not present. If _MQ present than get reliable data from kernel.
> > > > > > >>> If _MQ not present, it means this device has one VQ pair.
> > > > > > >> it is still a guess, right? And all user space tools implemented this
> > > > > > >> feature need to guess
> > > > > > > No. it is not a guess.
> > > > > > > It is explicitly checking the _MQ feature and deriving the value.
> > > > > > > The code you proposed will be present in the user space.
> > > > > > > It will be uniform for _MQ and 10 other features that are present now and
> > > > > > in the future.
> > > > > > MQ and other features like RSS are different. If there is no _RSS_XX, there
> > > > > > are no attributes like max_rss_key_size, and there is not a default value.
> > > > > > But for MQ, we know it has to be 1 wihtout _MQ.
> > > > > "we" = user space.
> > > > > To keep the consistency among all the config space fields.
> > > >
> > > > Actually I looked and the code some more and I'm puzzled:
> > > >
> > > >
> > > >         struct virtio_net_config config = {};
> > > >         u64 features;
> > > >         u16 val_u16;
> > > >
> > > >         vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> > > >
> > > >         if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
> > > >                     config.mac))
> > > >                 return -EMSGSIZE;
> > > >
> > > >
> > > > Mac returned even without VIRTIO_NET_F_MAC
> > > >
> > > >
> > > >         val_u16 = le16_to_cpu(config.status);
> > > >         if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
> > > >                 return -EMSGSIZE;
> > > >
> > > >
> > > > status returned even without VIRTIO_NET_F_STATUS
> > > >
> > > >         val_u16 = le16_to_cpu(config.mtu);
> > > >         if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
> > > >                 return -EMSGSIZE;
> > > >
> > > >
> > > > MTU returned even without VIRTIO_NET_F_MTU
> > > >
> > > >
> > > > What's going on here?
> > >
> > > Probably too late to fix, but this should be fine as long as all
> > > parents support STATUS/MTU/MAC.
> >
> > Why is this too late to fix.
> 
> If we make this conditional on the features. This may break the
> userspace that always expects VDPA_ATTR_DEV_NET_CFG_MTU?
> 
> Thanks

Well only on devices without MTU. I'm saying said userspace
was reading trash on such devices anyway.
We don't generally maintain bug for bug compatiblity on a whim,
only if userspace is actually known to break if we fix a bug.

> >
> > > I wonder if we can add a check in the core and fail the device
> > > registration in this case.
> > >
> > > Thanks
> > >
> > > >
> > > >
> > > > --
> > > > MST
> > > >
> >