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(-)
