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

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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





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