RE: [PATCH v5 10/13] vdpa: Support reporting max device virtqueues

[Parav Pandit via Virtualization]

> From: Eli Cohen <e...@nvidia.com>
> Sent: Wednesday, December 22, 2021 12:17 PM
> 
> > > > --- a/drivers/vdpa/vdpa.c
> > > > +++ b/drivers/vdpa/vdpa.c
> > > > @@ -507,6 +507,9 @@ static int vdpa_mgmtdev_fill(const struct
> > > vdpa_mgmt_dev *mdev, struct sk_buff *m
> > > >                 err = -EMSGSIZE;
> > > >                 goto msg_err;
> > > >         }
> > > > +       if (nla_put_u16(msg, VDPA_ATTR_DEV_MGMTDEV_MAX_VQS,
> > > > +                       mdev->max_supported_vqs))
> > > It still needs a default value when the field is not explicitly
> > > filled in by the driver.
> > >
> > Unlikely. This can be optional field to help user decide device max limit.
> > When max_supported_vqs is set to zero. Vdpa should omit exposing it to user
> space.
> >
> 
> This is not about what you expose to userspace. It's about the number of VQs
> you want to create for a specific instance of vdpa.
This value on mgmtdev indicates that a given mgmt device supports creating a 
vdpa device who can have maximum VQs of N.
User will choose to create VQ with VQs <= N depending on its vcpu and other 
factors.

This is what is exposed to the user to decide the upper bound.
> > There has been some talk/patches of rdma virtio device.
> > I anticipate such device to support more than 64K queues by nature of rdma.
> > It is better to keep max_supported_vqs as u32.
> 
> Why not add it when we have it?
Sure, with that approach we will end up adding two fields (current u16 and 
later another u32) due to smaller bit width of current one.
Either way is fine. Michael was suggesting similar higher bit-width in other 
patches, so bringing up here for this field on how he sees it.
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