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Date:   Thu, 30 Nov 2023 08:36:48 -0800
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Subject: Re: [PATCH v5] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT
From:   Sean Christopherson <seanjc@google.com>
To:     Paul Durrant <paul@xen.org>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        David Woodhouse <dwmw2@infradead.org>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andrew Cooper <andrew.cooper3@citrix.com>
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+Andrew

On Thu, Nov 02, 2023, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 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.