[Bug 1860053] [NEW] Possible lack of precision when calling clock_gettime via vDSO on user mode ppc64le

* [Bug 1860053] [NEW] Possible lack of precision when calling clock_gettime via vDSO on user mode ppc64le
@ 2020-01-16 23:32 Patrick Meiring
  2020-01-17  6:46 ` [Bug 1860053] " Philippe Mathieu-Daudé
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: Patrick Meiring @ 2020-01-16 23:32 UTC (permalink / raw)
  To: qemu-devel

Public bug reported:

Occurs on QEMU v4.2.0 run on docker (via the qemu-user-static:v4.2.0-2
image) on an AMD64 Ubuntu 18.04.3 LTS machine provided by travis-ci.org.

>From golang's https://github.com/golang/go/issues/36592:

It was discovered that golang's time.NewTicker() and time.Sleep()
malfunction when a compiled application was run via QEMU's ppc64le
emulator in user mode.

The methods did not malfunction on actual PowerPC hardware or when the
same golang application was compiled for golang's arm, arm64 or 386
targets and was run via user mode QEMU on the same system.

Curiously, the methods also worked when the program was compiled under
go 1.11, but do malfunction in go 1.12 and 1.13.

It was identified the change in behaviour was most likely attributable to golang switching to using vSDO for calling clock_gettime() on PowerPC 64 architectures in 1.12. I.E:
https://github.com/golang/go/commit/dbd8af74723d2c98cbdcc70f7e2801f69b57ac5b

We therefore suspect there may be a bug in QEMU's user-mode emulation of
ppc64le as relates to vDSO calls to clock_gettime().

The nature of the malfunction of time.NewTicker() and time.Sleep() is
such that sleeps or ticks with a granularity of less than one second do
not appear to be possible (they all revert to 1 second sleeps/ticks).
Could it be that the nanoseconds field of clock_gettime() is getting
lost in the vDSO version but not in the syscall? Or some other issue
calling these methods via vDSO?

Thanks in advance.

** Affects: qemu
     Importance: Undecided
         Status: New

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Title:
  Possible lack of precision when calling clock_gettime via vDSO on user
  mode ppc64le

Status in QEMU:
  New

Bug description:
  Occurs on QEMU v4.2.0 run on docker (via the qemu-user-static:v4.2.0-2
  image) on an AMD64 Ubuntu 18.04.3 LTS machine provided by travis-
  ci.org.

  From golang's https://github.com/golang/go/issues/36592:

  It was discovered that golang's time.NewTicker() and time.Sleep()
  malfunction when a compiled application was run via QEMU's ppc64le
  emulator in user mode.

  The methods did not malfunction on actual PowerPC hardware or when the
  same golang application was compiled for golang's arm, arm64 or 386
  targets and was run via user mode QEMU on the same system.

  Curiously, the methods also worked when the program was compiled under
  go 1.11, but do malfunction in go 1.12 and 1.13.

  It was identified the change in behaviour was most likely attributable to golang switching to using vSDO for calling clock_gettime() on PowerPC 64 architectures in 1.12. I.E:
  https://github.com/golang/go/commit/dbd8af74723d2c98cbdcc70f7e2801f69b57ac5b

  We therefore suspect there may be a bug in QEMU's user-mode emulation
  of ppc64le as relates to vDSO calls to clock_gettime().

  The nature of the malfunction of time.NewTicker() and time.Sleep() is
  such that sleeps or ticks with a granularity of less than one second
  do not appear to be possible (they all revert to 1 second
  sleeps/ticks). Could it be that the nanoseconds field of
  clock_gettime() is getting lost in the vDSO version but not in the
  syscall? Or some other issue calling these methods via vDSO?

  Thanks in advance.

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