[RFC PATCH v2] Cleaning up the KVM clock mess

[David Woodhouse]

Clean up the KVM clock mess somewhat so that it is either based on the guest
TSC ("master clock" mode), or on the host CLOCK_MONOTONIC_RAW in cases where
the TSC isn't usable.

Eliminate the third variant where it was based directly on the *host* TSC,
due to bugs in e.g. __get_kvmclock().

Kill off the last vestiges of the KVM clock being based on CLOCK_MONOTONIC
instead of CLOCK_MONOTONIC_RAW and thus being subject to NTP skew.

Fix up migration support to allow the KVM clock to be saved/restored as an
arithmetic function of the guest TSC, since that's what it actually is in
the *common* case so it can be migrated precisely. Or at least to within
±1 ns which is good enough, as discussed in
https://lore.kernel.org/kvm/c8dca08bf848e663f192de6705bf04aa3966e856.camel@xxxxxxxxxxxxx

In v2 of this series, TSC synchronization is improved and simplified a bit
too, and we allow masterclock mode to be used even when the guest TSCs are
out of sync, as long as they're running at the same *rate*. The different
*offset* shouldn't matter.

And the kvm_get_time_scale() function annoyed me by being entirely opaque,
so I studied it until my brain hurt and then added some comments.

In v2 I also dropped the commits which were removing the periodic clock
syncs. Those are going to be needed still but *only* for non-masterclock
mode, which I'll do next. Along with ensuring that a masterclock update
while already in masterclock mode doesn't jump the clock, and just does
the same as KVM_SET_CLOCK_GUEST does to preserve it.

Needs a *lot* more testing. I think I'm almost done refactoring the code,
so should focus on building up the tests next.

(I do still hate that we're abusing KVM_GET_CLOCK just to get the tuple
of {host_tsc, CLOCK_REALTIME} without even *caring* about the eponymous
KVM clock. Especially as this information is (a) fundamentally what the
vDSO gettimeofday() exposes to us anyway, (b) using CLOCK_REALTIME not
TAI, (c) not available on other platforms, for example for migrating
the Arm arch counter.)

David Woodhouse (13):
      KVM: x86/xen: Do not corrupt KVM clock in kvm_xen_shared_info_init()
      KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force
      KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC
      KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration
      KVM: x86: Avoid NTP frequency skew for KVM clock on 32-bit host
      KVM: x86: Fix KVM clock precision in __get_kvmclock()
      KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()
      KVM: x86: Simplify and comment kvm_get_time_scale()
      KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()
      KVM: x86: Improve synchronization in kvm_synchronize_tsc()
      KVM: x86: Kill cur_tsc_{nsec,offset,write} fields
      KVM: x86: Allow KVM master clock mode when TSCs are offset from each other
      KVM: x86: Factor out kvm_use_master_clock()

Jack Allister (2):
      KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration
      KVM: selftests: Add KVM/PV clock selftest to prove timer correction

 Documentation/virt/kvm/api.rst                    |  37 ++
 Documentation/virt/kvm/devices/vcpu.rst           | 115 +++-
 arch/x86/include/asm/kvm_host.h                   |  15 +-
 arch/x86/include/uapi/asm/kvm.h                   |   6 +
 arch/x86/kvm/svm/svm.c                            |   3 +-
 arch/x86/kvm/vmx/vmx.c                            |   2 +-
 arch/x86/kvm/x86.c                                | 687 +++++++++++++++-------
 arch/x86/kvm/xen.c                                |   4 +-
 include/uapi/linux/kvm.h                          |   3 +
 tools/testing/selftests/kvm/Makefile              |   1 +
 tools/testing/selftests/kvm/x86_64/pvclock_test.c | 192 ++++++
 11 files changed, 822 insertions(+), 243 deletions(-)