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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ivan Dyukov
> Sent: Thursday, April 2, 2020 2:54 PM
> 
> 01.04.2020 13:06, Morten Brørup пишет:
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas Monjalon
> >> Sent: Wednesday, April 1, 2020 11:54 AM
> >>
> >> 01/04/2020 11:33, Morten Brørup:
> >>> Thomas, Ferruh, Andrew (Ethernet API Maintainers),
> >>>
> >>> A command line option was recently added to set which speed a vNIC
> >> reports when the link is up. This makes sense for Spanning Tree and
> >> other protocols which depend on link speed.
> >>
> >> Please could you reference the patch?
> > It is a patch for the virtio driver:
> > https://protect2.fireeye.com/url?k=e37beb37-beabe3df-e37a6078-
> 000babff32e3-
> 4aaaa0986ed7ec57&u=http://inbox.dpdk.org/dev/20191212085012.9170-1-
> i.dyukov@samsung.com/T/#m052f90ea8c559406aeaefaea1fc24ed9bb573788
> This patch is related to similar work in qemu & kernel virtio driver.
> Please see
> https://lists.oasis-open.org/archives/virtio-
> comment/201911/msg00058.html.
> These changes have been implemented and released in kernel and qemu.
> speed is negotiated from qemu and then user may change the speed of
> virtio device using ethtool utility. I have added similiar patchset for
> pmd driver which do the same but for changing speed I used devargs
> instead of ethtool.

Very interesting link, indeed!

It gives the virtio driver the possibility to expose a specific speed in Mbit/s, which I assume - when used correctly - should reflect the speed of the underlying hardware. So it could be 20 Gbit/s for a link aggregate (in IEEE 802.3 Ethernet terminology; "bond" in Linux terminology) of two 10G ports.

It also provides a special value for unknown speed.

> >
> >>> However, I suspect that this workaround rarely reflects the
> physical
> >> truth, and suggest that the application should handle it instead.
> >>
> >> I don't understand why we need to define some speed for virtual
> >> devices.
> >>
> >>> In other words... Instead of faking it in the virtual Ethernet
> >> drivers, I suggest that rte_ethdev.h defines a special speed value
> for
> >> vNICs which really don't have a physical link speed:
> >>> #define ETH_SPEED_NUM_NONE         0 /**< Not defined */
> >> The only issue with this constant is the lack of RTE_ prefix :-)
> >> Otherwise I think "0 - NONE - not defined" fits well with virtual
> >> device case.
> >>
> >>> +#define ETH_SPEED_NUM_UNKNOWN      1 /**< Unknown (virtual device)
> >> */
> >>
> >> 1 means 1 Mbps
> >>
> >>> #define ETH_SPEED_NUM_10M         10 /**<  10 Mbps */
> >>>
> >>> Alternatively, we could expand the meaning of ETH_SPEED_NUM_NONE:
> >>>
> >>> -#define ETH_SPEED_NUM_NONE         0 /**< Not defined */
> >>> +#define ETH_SPEED_NUM_NONE         0 /**< Not defined or unknown
> >> (virtual device) */
> >>
> >> Yes I agree with extending the comment for NONE.
> >>
> >>> The special value could also be used in cases like this:
> >>>
> >> https://protect2.fireeye.com/url?k=6154668d-3c846e65-6155edc2-
> 000babff32e3-
> bf63d034253cac80&u=http://inbox.dpdk.org/dev/AM0PR0502MB401907ADE7CEA27
> DC642DF35D2CB0@AM0P
> >> R0502MB4019.eurprd05.prod.outlook.com/T/#t
> >>
> >> Yes, if speed is unknown, it should be reported as 0.

Could the DPDK vNIC PMDs be updated accordingly? At least the virtio driver...

> > So the next related question is: Should a vNIC be allowed to report a
> fake speed if it does not know that the underlying hardware actually
> provides this speed?

Your link to the Kernel/QEMU patch answers this question: Yes, because we assume it reflects the underlying speed.