[PATCH v6 1/3] KVM: fix steal clock warp during guest cpu hotplug

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

Sometimes, after CPU hotplug you can observe a spike in stolen time
(100%) followed by the CPU being marked as 100% idle when it's actually
busy with a CPU hog task.  The trace looks like the following:

cpuhp/1-12    [001] d.h1   167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0
cpuhp/1-12    [001] d.h1   167.461659: account_process_tick: steal_jiffies = 1291
<idle>-0     [001] d.h1   167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000
<idle>-0     [001] d.h1   167.462664: account_process_tick: steal_jiffies = 18446744072437

The sudden decrease of "steal" causes steal_jiffies to underflow.
The root cause is kvm_steal_time being reset to 0 after hot-plugging
back in a CPU.  Instead, the preexisting value can be used, which is
what the core scheduler code expects.

John Stultz also reported a similar issue after guest S3.

Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 arch/x86/kernel/kvm.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index eea2a6f..1ef5e48 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -301,8 +301,6 @@ static void kvm_register_steal_time(void)
 	if (!has_steal_clock)
 		return;
 
-	memset(st, 0, sizeof(*st));
-
 	wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
 	pr_info("kvm-stealtime: cpu %d, msr %llx\n",
 		cpu, (unsigned long long) slow_virt_to_phys(st));
-- 
1.9.1