[Xen-devel] [PATCH v3 0/6] x86/time: PVCLOCK_TSC_STABLE_BIT support

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

Hey!

This is v3 on the pvclock TSC stable bit series.

Complete changelog on individual patches but overall is addressing
Jan's comments, plus some other changes regarding the recent monotonicity
improvements on x86/time.

Series is divided as follows:

 R      * Patch 1: Small refactor around init_platform_time to reuse
                   initialization code when switching to TSC.
 U      * Patch 2: Adds a new clocksource based on TSC
 U,U    * Patch 3, 4: Adjustments for patch 5
 U      * Patch 5: Implements the PVCLOCK_TSC_STABLE_BIT
 N      * Patch 6: Document new clocksource

[ R := Reviewed-by ;; U := Updated ;; N := New ]

I kept the series the same but a fundamental difference from previous
versions is that I stop clocksource=tsc from being used at all if hotplug is
possible. To facilitate the review I kept it on Patch 5 as originally posted,
whereas clocksource is added in Patch 2. But if preferred I can merge these two.

The main benefit of this series is two-fold:
 
 1. Provide the guarantee of monotonic results on xen own system time as seen
 by any cpu when using TSC as clocksource.

 2. Provide this same guarantee to guests and thus set the
 TSC_STABLE_BIT (both FreeBSD and Linux support it) which then allows guests to
 skip expensive monotonicity check between PV CPU time infos. Plus, on Linux
 specifically this also means that it could support vDSO which greatly increases
 performance (x10) for gettimeofday and clock_gettime since it would no
 longer need to do the system call to get a reliable snapshot of system time.
 For a reference on my laptop the speed of gettimeofday under xen pvclock is 
 ~2 Mops/sec (Million ops per sec) whereas with vDSO it's on the range
 of ~22 Mops/sec on <= 4.4 kernels and ~37 Mops on >= 4.5.
 
 Doing a long running time warp test for the past days on a dual-socket Haswell
 machine and I haven't yet seen time going backwards.

Thanks!
Joao

Joao Martins (6):
  x86/time: refactor init_platform_time()
  x86/time: implement tsc as clocksource
  x86/time: streamline platform time init on plt_update()
  x86/time: refactor read_platform_stime()
  x86/time: implement PVCLOCK_TSC_STABLE_BIT
  docs: update clocksource option

 docs/misc/xen-command-line.markdown |   6 +-
 xen/arch/x86/platform_hypercall.c   |   3 +-
 xen/arch/x86/time.c                 | 226 +++++++++++++++++++++++++++++++-----
 xen/include/asm-x86/time.h          |   1 +
 4 files changed, 205 insertions(+), 31 deletions(-)

-- 
2.1.4


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