svn commit: r325506 - in head: sbin/ifconfig sys/net sys/sys
John Baldwin
jhb at freebsd.org
Tue Nov 7 22:10:56 UTC 2017
On Tuesday, November 07, 2017 07:44:31 PM Konstantin Belousov wrote:
> On Tue, Nov 07, 2017 at 07:39:26PM +0200, Konstantin Belousov wrote:
> > On Tue, Nov 07, 2017 at 09:06:52AM -0800, John Baldwin wrote:
> > > On Tuesday, November 07, 2017 09:29:15 AM Konstantin Belousov wrote:
> > > > Author: kib
> > > > Date: Tue Nov 7 09:29:14 2017
> > > > New Revision: 325506
> > > > URL: https://svnweb.freebsd.org/changeset/base/325506
> > > >
> > > > Log:
> > > > Add a place for a driver to report rx timestamps in nanoseconds from
> > > > boot for the received packets.
> > > >
> > > > The rcv_tstmp field overlaps the place of Ln header length indicators,
> > > > not used by received packets. The basic pkthdr rearrangement change
> > > > in sys/mbuf.h was provided by gallatin.
> > > >
> > > > There are two accompanying M_ flags: M_TSTMP means that there is the
> > > > timestamp (and it was generated by hardware).
> > > >
> > > > Another flag M_TSTMP_HPREC indicates that the timestamp is
> > > > high-precision. Practically M_TSTMP_HPREC means that hardware
> > > > provided additional precision comparing with the stamps when the flag
> > > > is not set. E.g., for ConnectX all packets are stamped by hardware
> > > > when PCIe transaction to write out the completion descriptor is
> > > > performed, but PTP packet are stamped on port. For Intel cards, when
> > > > PTP assist is enabled, only PTP packets are stamped in the limited
> > > > number of registers, so if Intel cards ever start support this
> > > > mechanism, they would always set M_TSTMP | M_TSTMP_HPREC if hardware
> > > > timestamp is present for the given packet.
> > > >
> > > > Add IFCAP_HWRXTSTMP interface capability to indicate the support for
> > > > hardware rx timestamping, and ifconfig(8) command to toggle it.
> > >
> > > Hmm, other NICs (Chelsio T4 and later for example) support timestamps that
> > > aren't in nanoseconds but some other frequency (which are themselves useful).
> > > It would be nice to have a more flexible interface that supports not only ns
> > > timestamps. Perhaps a way to expose a direct hardware timestamp as a
> > > "number" without a specific frequency?
> >
> > ConnectX does not provide ns-clocked counter either. It is some internal
> > clock driven by a cristal with > 100MHz frequency.
> >
> > There is no much space in the pkthdr, and the request to provide the
> > timestamp was in the context where the wall clock or some closely related
> > timer is needed. Of course, I can put raw hardware timestamp into the
> > packet header, but only instead of the reduced value. Then the consumer
> > of the timestamp would need to find the interface which received the
> > packet and call its method to convert ? We have only one consumer in
> > tree (SO_TIMESTAMP) and perhaps one possible another consumer (TCP) for
> > this data, both of which require wall clock, so would need to call into
> > the method.
> >
> > Also please see the discussion in the referenced review about accuracy of
> > the convertion.
> >
> > Important example are Intel cards where is only limited number of
> > latched registers, and only PtP packets are stamped. This (and some
> > quirk in ConnectX) explains the high-precision flag.
> >
>
> And another consideration which was one of the strong argument for me
> when I thought about this stuff: the convertion of the hardware timestamp
> to the useful clock stamp depends on the clock calibraton data which might
> not be available long time after the packet receive. In other words, when
> the consumer would call into the interface method to convert raw timestamp,
> it might be already not convertable (in kern_tc.c terms, timehands were
> switched by tc_windup()).
When using the timestamps from Chelsio NICs previously, the codebase in
question performed conversions in userland rather than in the kernel using
other application-specific data in the received packets to aid in mapping
the timestamp values to wall times. Reliably converting a random timestamp
from the hardware to a wall time is indeed non-trivial. I had imagined
having a variant of SO_TIMESTAMP (or perhaps a new option) so that userland
could request the raw timestamp.
To be clear, however, is the MLX5 timestamp steered by an on-card PTP
implementation or is it just a free-running timer? (The Chelsio timer is
a free-running timer.)
--
John Baldwin
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