libc: Fall back to rdtsc when using pvclock and rdtscp is not available
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Authored by markj on Feb 1 2023, 5:17 PM.

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rGdadfe099255d: libc: Fall back to rdtsc when using pvclock and rdtscp is not available
rG26d105199ec8: libc: Fall back to rdtsc when using pvclock and rdtscp is not available

Summary

Now that kvmclock can optionally export timekeeping info to userspace
even on systems without rdtscp, we should fall back to using rdtsc when
necessary. This mimics pvclock_read_time_info() in the kernel.

Tested by: Shrikanth R Kamath <kshrikanth@juniper.net>

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rG FreeBSD src repository

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Buildable 49416
Build 46306: arc lint + arc unit

Event Timeline

markj created this revision.Feb 1 2023, 5:17 PM

Herald added a subscriber: imp. · View Herald TranscriptFeb 1 2023, 5:17 PM

markj requested review of this revision.Feb 1 2023, 5:17 PM

Harbormaster completed remote builds in B49416: Diff 116206.Feb 1 2023, 5:17 PM

markj added a child revision: D38342: pvclock: Export a vDSO page even without rdtscp available.Feb 1 2023, 5:18 PM

markj mentioned this in D38343: man4: Add a manual page for kvmclock.Feb 1 2023, 5:31 PM

I am curious, what is the hw/sw configuration where RDTSCP is not available? I believe the instruction itself was added at least with Nehalem (2009).

This revision is now accepted and ready to land.Feb 1 2023, 8:30 PM

In D38341#871021, @kib wrote:

I am curious, what is the hw/sw configuration where RDTSCP is not available? I believe the instruction itself was added at least with Nehalem (2009).

By default, the machine emulated by qemu-system-x86_64 is missing many "newer" capabilities. I did not ask for the exact configuration, but in my testing, the default -machine and -cpu parameters give you a system without AMDID_RDTSCP. So, this diff is mostly useful in cases where one does not control the (poorly chosen) hypervisor configuration.

Closed by commit rG26d105199ec8: libc: Fall back to rdtsc when using pvclock and rdtscp is not available (authored by markj). · Explain WhyFeb 3 2023, 4:51 PM

This revision was automatically updated to reflect the committed changes.

markj added a commit: rG26d105199ec8: libc: Fall back to rdtsc when using pvclock and rdtscp is not available.

markj added a commit: rGdadfe099255d: libc: Fall back to rdtsc when using pvclock and rdtscp is not available.Feb 20 2023, 4:24 PM