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
