Date: Tue, 07 Nov 2017 11:00:33 -0800 From: John Baldwin <jhb@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r325506 - in head: sbin/ifconfig sys/net sys/sys Message-ID: <5168224.cmPK861r6s@ralph.baldwin.cx> In-Reply-To: <20171107174431.GQ2566@kib.kiev.ua> References: <201711070929.vA79TFTc096109@repo.freebsd.org> <20171107173926.GP2566@kib.kiev.ua> <20171107174431.GQ2566@kib.kiev.ua>
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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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