On Thu, Nov 30, 2023, David Woodhouse wrote: > On Thu, 2023-11-30 at 16:41 +0000, Paul Durrant wrote: > > On 30/11/2023 16:36, Sean Christopherson wrote: > > > +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. > > > > It's certainly tested in arch/x86/xen/time.c, in > > xen_setup_vsyscall_time_info() and xen_time_init(), so I'd guess it is > > considered to be supported. > > And yes, Xen does set it, if you jump through the right hoops to make > Xen actually use the TSC as its clocksource. > > The new uAPI is just a single bit in the KVM_XEN_HVM_CONFIG > capabilities; I think it's reasonable enough. Yeah, I was just hoping that maybe we could squeak by without it. I'll get this queued up next week, purely because I try to avoid (but often fail) pushing to -next on Fridays.
