RE: [PATCH v2 4/4] Add support for MSI-X vectors configuration for PCI VFs

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Friday, January 28, 2022 8:54 AM
> vxlan part actually.

Right.
> 
> >
> > Similarly, we prefer the ability and flexibility to set more fields in one
> command.
> 
> I understood this motivation. But what I want to say is:
> 
> struct virtio_cmd {
>     u64 attr_mask;
>     field_X;
>     field_Y;
>     field_Z;
> }
> 
> What's the default value if one of the X,Y, Z is not specified? 
If minimum default values are not provided, HV cannot proceed to do configuration and ENOSUPPORT error code is returned by kernel to user.

> How to get those default values?
>
By doing identify or _get respective command.
 
> And suppose we add a new field
> 
> struct virtio_cmd {
>     u64 attr_mask;
>     field_X;
>     field_Y;
>     field_Z;
>     field_M;
> }
> 
> How can the driver know it can use field_M? 
If attribute mask is present for field_M, driver can use it. 

> And if field_M is supported in src
> but not dst, how can we keep the migration compatibility?
This is HV level command to query and provision a VF.
Migration compatibility is high level check where it will identify on which HV to migrate where I can provision X vectors for a VF.

> 
> Would it be simple to just mandate all the fields in this case?
>
May be, attribute mask is basically to let expand structure without inventing new fields.
For a given command few minimal attributes to be set and driver can verify, that if they are not, a given functionality is unsupported.