On Thu, Sep 16, 2021 at 06:15:35PM +0000, Oliver Upton wrote: > Handling the migration of TSCs correctly is difficult, in part because > Linux does not provide userspace with the ability to retrieve a (TSC, > realtime) clock pair for a single instant in time. In lieu of a more > convenient facility, KVM can report similar information in the kvm_clock > structure. > > Provide userspace with a host TSC & realtime pair iff the realtime clock > is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid > realtime value, advance the KVM clock by the amount of elapsed time. Do > not step the KVM clock backwards, though, as it is a monotonic > oscillator. > > Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx> > --- > Documentation/virt/kvm/api.rst | 42 ++++++++++++++++++++++++++------- > arch/x86/include/asm/kvm_host.h | 3 +++ > arch/x86/kvm/x86.c | 36 +++++++++++++++++++++------- > include/uapi/linux/kvm.h | 7 +++++- > 4 files changed, 70 insertions(+), 18 deletions(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index a6729c8cf063..d0b9c986cf6c 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -993,20 +993,34 @@ such as migration. > When KVM_CAP_ADJUST_CLOCK is passed to KVM_CHECK_EXTENSION, it returns the > set of bits that KVM can return in struct kvm_clock_data's flag member. > > -The only flag defined now is KVM_CLOCK_TSC_STABLE. If set, the returned > -value is the exact kvmclock value seen by all VCPUs at the instant > -when KVM_GET_CLOCK was called. If clear, the returned value is simply > -CLOCK_MONOTONIC plus a constant offset; the offset can be modified > -with KVM_SET_CLOCK. KVM will try to make all VCPUs follow this clock, > -but the exact value read by each VCPU could differ, because the host > -TSC is not stable. > +FLAGS: > + > +KVM_CLOCK_TSC_STABLE. If set, the returned value is the exact kvmclock > +value seen by all VCPUs at the instant when KVM_GET_CLOCK was called. > +If clear, the returned value is simply CLOCK_MONOTONIC plus a constant > +offset; the offset can be modified with KVM_SET_CLOCK. KVM will try > +to make all VCPUs follow this clock, but the exact value read by each > +VCPU could differ, because the host TSC is not stable. > + > +KVM_CLOCK_REALTIME. If set, the `realtime` field in the kvm_clock_data > +structure is populated with the value of the host's real time > +clocksource at the instant when KVM_GET_CLOCK was called. If clear, > +the `realtime` field does not contain a value. > + > +KVM_CLOCK_HOST_TSC. If set, the `host_tsc` field in the kvm_clock_data > +structure is populated with the value of the host's timestamp counter (TSC) > +at the instant when KVM_GET_CLOCK was called. If clear, the `host_tsc` field > +does not contain a value. If the host TSCs are not stable, then KVM_CLOCK_HOST_TSC bit (and host_tsc field) are ambiguous. Shouldnt exposing them be conditional on stable TSC for the host ?
