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Message-ID: <1facc6e797ec42dcf6a027a4343c695a61e251f5.camel@infradead.org>
Subject: Re: [PATCH] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paul Durrant <paul@xen.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        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>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Joao Martins <joao.m.martins@oracle.com>
Date:   Tue, 10 Oct 2023 18:32:16 +0100
In-Reply-To: <20231010094047.3850928-1-paul@xen.org>
References: <20231010094047.3850928-1-paul@xen.org>
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On Tue, 2023-10-10 at 09:40 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
>=20
> Unless explicitly told to do so (by passing 'clocksource=3Dtsc' and
> 'tsc=3Dstable: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.


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