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

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Message-ID: <20250227021855.3257188-25-seanjc@google.com>
Date: Wed, 26 Feb 2025 18:18:40 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: 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, 
	"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: [PATCH v2 24/38] timekeeping: Resume clocksources before reading
 persistent clock

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>
---
 kernel/time/timekeeping.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 1e67d076f195..332d053fa9ce 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1794,11 +1794,16 @@ void timekeeping_resume(void)
 	u64 cycle_now, nsec;
 	unsigned long flags;
 
-	read_persistent_clock64(&ts_new);
-
 	clockevents_resume();
 	clocksource_resume();
 
+	/*
+	 * Read persistent time after clocksources have been resumed.  Paravirt
+	 * clocks have a nasty habit of piggybacking a persistent clock on a
+	 * system clock, and may return garbage if the system clock is suspended.
+	 */
+	read_persistent_clock64(&ts_new);
+
 	raw_spin_lock_irqsave(&tk_core.lock, flags);
 
 	/*
-- 
2.48.1.711.g2feabab25a-goog