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Subject[PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug
Date
From: Wanpeng Li <wanpeng.li@hotmail.com>

I observed that sometimes st is 100% instantaneous, then idle is 100%
even if there is a cpu hog on the guest cpu after the cpu hotplug comes
back(N.B. this can not always be readily reproduced). I add trace to
capture it as below:

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 steal clock warp and then steal_jiffies underflow.

Rik also pointed out to me:

| I have seen stuff like that with live migration too, in the past

The root cause of steal clock warp during hotplug is kvm_steal_time reset
to 0 after cpu hotplug comes back which should be preexiting guest value.
This patch fix it by don't reset kvm_steal_time during guest cpu hotplug.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v2 -> v3:
* fix the root cause
v1 -> v2:
* update patch subject, description and comments
* deal with the case where steal time suddenly increases by a ludicrous amount

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
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