+Andrew On Thu, Nov 02, 2023, Paul Durrant wrote: > From: Paul Durrant <pdurrant@xxxxxxxxxx> > > Unless explicitly told to do so (by passing 'clocksource=tsc' and > 'tsc=stable:socket', and then jumping through some hoops concerning > potential CPU hotplug) Xen will never use TSC as its clocksource. > Hence, by default, a Xen guest will not see PVCLOCK_TSC_STABLE_BIT set > in either the primary or secondary pvclock memory areas. This has > led to bugs in some guest kernels which only become evident if > PVCLOCK_TSC_STABLE_BIT *is* set in the pvclocks. Hence, to support > such guests, give the VMM a new Xen HVM config flag to tell KVM to > forcibly clear the bit in the Xen pvclocks. ... > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 7025b3751027..a9bdd25826d1 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -8374,6 +8374,7 @@ PVHVM guests. Valid flags are:: > #define KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL (1 << 4) > #define KVM_XEN_HVM_CONFIG_EVTCHN_SEND (1 << 5) > #define KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG (1 << 6) > + #define KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE (1 << 7) Does Xen actually support PVCLOCK_TSC_STABLE_BIT? I.e. do we need new uAPI to fix this, or can/should KVM simply _never_ set PVCLOCK_TSC_STABLE_BIT for Xen clocks? At a glance, PVCLOCK_TSC_STABLE_BIT looks like it was added as a purely Linux/KVM-only thing.
