Due to unintialized kvmclock read KVM guest is hanging on SMP boot stage.
If unintialized memory contains fatal garbage then hang reproduction is 100%.
Unintialized memory is allocated by memblock_alloc. So the garbage values
depend on many many things.
See the detailed description of the bug and possible ways to fix it
in the kernel bug tracker.
"Bug 59521 - KVM linux guest reads uninitialized pvclock values before
executing rdmsr MSR_KVM_WALL_CLOCK"
I decided to fix it simply returning 0ULL from kvm_clock_read when
kvm clocksource is not initialized yet.
The same as kernel bootstrap CPU doesn on boot stage when kernel
clocksources are not initialized yet.
Signed-off-by: Eugene Batalov <ebatalov@xxxxxxxxxxxxx>
---
I dont' use kernel percpu variables because for each SMP CPU
their contents are copied from the bootstrap CPU. And I don't
think that fixing the value for each SMP CPU is a good style.
If you know a better approach to store the is_pv_clock_ready flags
I am ready to use it.
The patch applies cleanly to
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
I've tested the changes with Ubuntu 13.04 "raring" userspace and
Ubuntu-3.8.0.19-30 kernel tag.
arch/x86/kernel/kvmclock.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 5bedbdd..a6e0af4 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -43,6 +43,9 @@ early_param("no-kvmclock", parse_no_kvmclock);
static struct pvclock_vsyscall_time_info *hv_clock;
static struct pvclock_wall_clock wall_clock;
+/* For each cpu store here a flag which tells whether pvclock is initialized */
+static int __cacheline_aligned_in_smp is_pv_clock_ready[NR_CPUS] = {};
+
/*
* The wallclock is the time of day when we booted. Since then, some time may
* have elapsed since the hypervisor wrote the data. So we try to account for
@@ -84,8 +87,11 @@ static cycle_t kvm_clock_read(void)
preempt_disable_notrace();
cpu = smp_processor_id();
- src = &hv_clock[cpu].pvti;
- ret = pvclock_clocksource_read(src);
+ if (is_pv_clock_ready[cpu]) {
+ src = &hv_clock[cpu].pvti;
+ ret = pvclock_clocksource_read(src);
+ } else
+ ret = 0ULL;
preempt_enable_notrace();
return ret;
}
@@ -168,6 +174,9 @@ int kvm_register_clock(char *txt)
printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n",
cpu, high, low, txt);
+ if (!ret)
+ is_pv_clock_ready[cpu] = 1;
+
return ret;
}
--
1.7.9.5
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