RE: [PATCH v3] virtio-net: support the virtqueue coalescing moderation

[Parav Pandit <parav@nvidia.com>]

> From: Heng Qi <hengqi@linux.alibaba.com>
> Sent: Friday, February 17, 2023 12:24 AM

> I think there are two kinds of sequences:
> #Seq1:
> 1. vq is disabled (vq params are reset to max_packets = 0, max_usecs = 0) 2.
> VQ_SET command arrives with max_packets=10, max_usecs = 8 3. vq is enabled
> (vq params are max_packets = 0, max_usecs = 0) After #3 is done, vq params
> should be max_packets = 0, max_usecs = 0, because the device cannot apply
> parameters when vq is disabled.
> Other reason is that parameters determined based on the previous traffic
> information before vq is re-enabled, and it is not applicable now.
From dev point of view, above is not a valid sequence.
VQ is the object whose lifecycle (create/delete/query/modify) is triggered by the driver.
So step 2 cannot happen before step 3.
If driver does that, driver should get error code INVALID_VQ on step 2.

> 
> #Seq1:
> 1. vq is disabled (vq params are reset to max_packets = 0, max_usecs = 0) 2. vq
> is enabled (vq params are max_packets = 0, max_usecs = 0) 3. VQ_SET
> command arrives with max_packets=10, max_usecs = 8 After 3 is done, vq
> params should be max_packets = 10, max_usecs = 8
> 
> Thanks.
> 
> >
> > > +
> > > +A device MAY NOT set the coalescing parameter to the exact same
> > > +value as the one passed in by a driver. For example, the value of
> \field{max_packets} set by the driver is 15, but the device MAY store a value
> that is a power of 2, that is, 16.
> > > +
> > >  A device SHOULD NOT send used buffer notifications to the driver, if the
> notifications are suppressed as explained in \ref{sec:Basic Facilities of a Virtio
> Device / Virtqueues / Used Buffer Notification Suppression}, even if the
> coalescing counters expired.
> > >
> > >  Upon reset, a device MUST initialize all coalescing parameters to 0.
> > > --
> > > 2.19.1.6.gb485710b