Re: [virtio-comment] [PATCH requirements 6/7] net-features: Add packet timestamp requirements

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 06, 2023 at 10:51:25PM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, June 6, 2023 6:40 PM
> 
> > > +## 3.5 Packet timestamp
> > > +1. Device should provide transmit timestamp and receive timestamp of the
> > packets
> > > +   at per packet level when the device is enabled.
> > > +2. Device should provide the current free running clock in the least latency
> > > +   possible using an MMIO register read of 64-bit to have the least jitter.
> > 
> > wow these reads are expensive. what is the actual requirement?
> > 
> I don't understand the question.

placing something in a register is a solution. what is the
requirement?

> > > +3. Device should provide the current frequency and the frequency unit for
> > the
> > > +   software to synchronize the reference point of software and the device
> > using
> > > +   a control vq command.
> > 
> > let's leave mechanism out of it. I think you are trying to say this is async to data
> > vqs? and cvq is fine.
> >
> There are two clocks running, one of cpu where driver is running, other one is in the device.
> And timestamps are done by the device based on its clock.
> 
> So driver can convert the device timestamp to driver cpu timestamp.
> This is like one time query to know conversion mechanics.
> 
> > > +### 3.5.2 Receive timestamp
> > > +1. Receive completion must contain a packet reception timestamp when the
> > device
> > > +   is enabled for it.
> > > +2. The device should record the received packet timestamp at the closet
> > ingress
> > > +   point of reception from the network.
> > > +3. The device should provide a receive packet timestamp in a single DMA
> > > +   transaction along with the rest of the receive completion fields.
> > 
> > how large do these need to be? ok for timer to overflow?
> >
> Will add it. Usually 8B.
> But overflow after 30+ years or so is common norm.
> 
>  
> > 
> > why do you mention migration in counters but not in timers?
> > 
> > is it ok for time to go back?
> 
> Should add it for timer too.