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@i...
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(-)