linux-kernel - Re: [PATCH v2 24/38] timekeeping: Resume clocksources before reading persistent clock

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Message-ID: <87r03jeska.ffs@tglx>
Date: Thu, 27 Feb 2025 08:48:53 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Juergen Gross <jgross@...e.com>, "K. Y. Srinivasan"
 <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu
 <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, Ajay Kaher
 <ajay.kaher@...adcom.com>, Jan Kiszka <jan.kiszka@...mens.com>, Andy
 Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>, John Stultz
 <jstultz@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
 kvm@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org, Tom Lendacky
 <thomas.lendacky@....com>, Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH v2 24/38] timekeeping: Resume clocksources before
 reading persistent clock

On Wed, Feb 26 2025 at 18:18, Sean Christopherson wrote:

> When resuming timekeeping after suspend, restore clocksources prior to
> reading the persistent clock.  Paravirt clocks, e.g. kvmclock, tie the
> validity of a PV persistent clock to a clocksource, i.e. reading the PV
> persistent clock will return garbage if the underlying PV clocksource
> hasn't been enabled.  The flaw has gone unnoticed because kvmclock is a
> mess and uses its own suspend/resume hooks instead of the clocksource
> suspend/resume hooks, which happens to work by sheer dumb luck (the
> kvmclock resume hook runs before timekeeping_resume()).
>
> Note, there is no evidence that any clocksource supported by the kernel
> depends on a persistent clock.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>