Re: [PATCH v5] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT

[Paul Durrant]

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.

  Paul