linux-kernel - Re: [PATCH v5] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE

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Message-ID: <356a88e424a58990a1b83afa719662e75f42bf98.camel@infradead.org>
Date:   Wed, 08 Nov 2023 18:39:59 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....org>, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] KVM x86/xen: add an override for
 PVCLOCK_TSC_STABLE_BIT

On Thu, 2023-11-02 at 16:21 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@...zon.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.
> 
> Signed-off-by: Paul Durrant <pdurrant@...zon.com>

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

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