linux-kernel - [PATCH 5.0 207/238] x86/kvmclock: set offset for kvm unstable clock

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Message-Id: <20190322111310.446706040@linuxfoundation.org>
Date:   Fri, 22 Mar 2019 12:17:06 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.0 207/238] x86/kvmclock: set offset for kvm unstable clock

5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Pavel Tatashin <pasha.tatashin@...een.com>

commit b5179ec4187251a751832193693d6e474d3445ac upstream.

VMs may show incorrect uptime and dmesg printk offsets on hypervisors with
unstable clock. The problem is produced when VM is rebooted without exiting
from qemu.

The fix is to calculate clock offset not only for stable clock but for
unstable clock as well, and use kvm_sched_clock_read() which substracts
the offset for both clocks.

This is safe, because pvclock_clocksource_read() does the right thing and
makes sure that clock always goes forward, so once offset is calculated
with unstable clock, we won't get new reads that are smaller than offset,
and thus won't get negative results.

Thank you Jon DeVree for helping to reproduce this issue.

Fixes: 857baa87b642 ("sched/clock: Enable sched clock early")
Cc: stable@...r.kernel.org
Reported-by: Dominique Martinet <asmadeus@...ewreck.org>
Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kernel/kvmclock.c |    6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -104,12 +104,8 @@ static u64 kvm_sched_clock_read(void)

 static inline void kvm_sched_clock_init(bool stable)
 {
-	if (!stable) {
-		pv_ops.time.sched_clock = kvm_clock_read;
+	if (!stable)
 		clear_sched_clock_stable();
-		return;
-	}
-
 	kvm_sched_clock_offset = kvm_clock_read();
 	pv_ops.time.sched_clock = kvm_sched_clock_read;