Re: [dpdk-dev] [RFC] ethdev: use special speed for virtual Ethernetdevices

[Thomas Monjalon <thomas@monjalon.net>]

03/04/2020 11:45, Morten Brørup:
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ivan Dyukov
> > Sent: Friday, April 3, 2020 10:06 AM
> > 
> > 02.04.2020 23:58, Thomas Monjalon пишет:
> > > 02/04/2020 22:41, Ivan Dyukov:
> > >> 02.04.2020 16:50, Morten Brørup пишет:
> > >>>>>> Yes, if speed is unknown, it should be reported as 0.
> > >>> Could the DPDK vNIC PMDs be updated accordingly? At least the
> > virtio driver...
> > >> Current version of dpdk code on master always returns 10G speed for
> > >> virtio device and many application rely on it. e.g. pktgen. If we'll
> > >> change it, we break the apps.
> > > I am OK with breaking such strange assumption.
> > > I can understand the need for specifying the underlying hardware
> > speed
> > > through virtio driver. But hardcoded 10G... no!
> > >
> > >
> > >
> > OK. I'll redefine it to 0xffffffff, like in kernel virtio.
> > 
> 
> Thomas, you were opposed to using 1 as the special value for "unknown", as it is likely interpreted as 1 Mbps instead, and I agree with your reasoning on that.
> 
> Since using an extremely large value is as close to infinity we can get, Ivan's suggested value makes sense to me instead:
> 
> +#define ETH_SPEED_NUM_UNKNOWN      0xffffffff /**< Unknown */
> 
> In theory, it also addresses Stephen's concern about breaking the ABI for existing applications that look at speed. They will get a non-zero speed, which is not a randomly chosen fake 10G speed; so I consider it an improvement.
> Although in reality, such applications may still break if they are unable to handle the extreme speed of 4.3 Pbps.
> 
> I considered "Unlimited" instead of "Unknown", but Unlimited is not really correct, so I settled with Unknown.

Yes it makes sense.
The only drawback is that the information "speed is accurate" will
be checked against two constants:
	ETH_SPEED_NUM_NONE or ETH_SPEED_NUM_UNKNOWN
It can be mitigated with a function though.