[x86] kvmclock: set offset for kvm unstable clock

Closes: #918036

debian/changelog

@@ -102,6 +102,9 @@ linux (4.19.17-1) UNRELEASED; urgency=medium
   * [arm64] enable ARM_CCI_PMU so ARM_CCI400_PMU and ARM_CCI5xx_PMU options
     get really enabled.
 
+  [ Salvatore Bonaccorso ]
+  * [x86] kvmclock: set offset for kvm unstable clock (Closes: #918036)
+
  -- Luca Boccassi <bluca@debian.org>  Fri, 18 Jan 2019 19:09:06 +0000
 
 linux (4.19.16-1) unstable; urgency=medium

debian/patches/bugfix/x86/x86-kvmclock-set-offset-for-kvm-unstable-clock.patch

@@ -0,0 +1,45 @@
+From: Pavel Tatashin <pasha.tatashin@soleen.com>
+Date: Sat, 26 Jan 2019 12:49:56 -0500
+Subject: x86/kvmclock: set offset for kvm unstable clock
+Origin: https://patchwork.kernel.org/patch/10782557/
+Bug-Debian: https://bugs.debian.org/918036
+
+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")
+
+Reported-by: Dominique Martinet <asmadeus@codewreck.org>
+Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
+[carnil: Backport to 4.19 for context changes]
+---
+ 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
+@@ -117,11 +117,8 @@ static u64 kvm_sched_clock_read(void)
+ 
+ static inline void kvm_sched_clock_init(bool stable)
+ {
+-	if (!stable) {
+-		pv_time_ops.sched_clock = kvm_clock_read;
++	if (!stable)
+ 		clear_sched_clock_stable();
+-		return;
+-	}
+ 
+ 	kvm_sched_clock_offset = kvm_clock_read();
+ 	pv_time_ops.sched_clock = kvm_sched_clock_read;

debian/patches/series

@@ -79,6 +79,7 @@ bugfix/arm64/arm64-acpi-Add-fixup-for-HPE-m400-quirks.patch
 bugfix/x86/x86-32-disable-3dnow-in-generic-config.patch
 bugfix/powerpc/powerpc-fix-mcpu-options-for-spe-only-compiler.patch
 bugfix/m68k/m68k-build-with-ffreestanding.patch
+bugfix/x86/x86-kvmclock-set-offset-for-kvm-unstable-clock.patch
 
 # Arch features
 features/mips/MIPS-increase-MAX-PHYSMEM-BITS-on-Loongson-3-only.patch