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

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Date:   Wed, 11 Oct 2023 18:11:25 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.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
Cc:     Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [PATCH] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT

On 10/10/2023 18:32, David Woodhouse wrote:
> On Tue, 2023-10-10 at 09:40 +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 pvclock.
> 
> Specifically, some OL7 kernels backported the whole pvclock vDSO thing
> but *forgot* https://git.kernel.org/torvalds/c/9f08890ab and thus kill
> init with a SIGBUS the first time it tries to read a clock, because
> they don't actually map the pvclock pages to userspace :)
> 
> They apparently never noticed because evidently *their* Xen fleet
> doesn't actually jump through all those hoops to use the TSC as its
> clocksource either.
> 
> It's a fairly safe bet that there are more broken guest kernels out
> there too, hence needing to work around it.
> 
>>   Hence, to support
>> such guests, give the VMM a new attribute to tell KVM to forcibly
>> clear the bit in the Xen pvclocks.
> 
> I frowned at the "PVCLOCK" part of the new attribute for a while,
> thinking that perhaps if we're going to have a set of flags to tweak
> behaviour, we shouldn't be so specific. Call it 'XEN_FEATURES' or
> something... but then I realised we'd want to *advertise* the set of
> bits which is available for userspace to set...
> 
> ... and then I realised we already do. That's exactly what the set of
> bits returned, and *set*, with KVM_CAP_XEN_HVM is for.
> 
> So let's ditch the new *attribute*, and just add your new (renamed)
> KVM_XEN_HVM_CONFIG_PVCLOCK_NO_STABLE_TSC cap to the set of
> permitted_flags in kvm_xen_hvm_config() so that userspace can enable it
> that way like it does the INTERCEPT_HYPERCALL and EVTCHN_SEND
> behaviours.
> 

Ok, sounds like a plan. I'll look at configuring it that way instead.

   Paul