[RFC PATCH 09/10] KVM: VMX: fix unsyc vmcs status when cpu is goingdown

From: Srivatsa S. Bhat
Date: Tue Dec 04 2012 - 03:57:25 EST

From: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>

When vcpu is scheduled to the different cpu, it should sent IPI to
the cpu which the vcpu ran to clear the vcpu->vmcs

It is safe since cpu-offline path can not concurrently run with
other cpu. After implementing stop_machine()-free, smp_call_function_sing
will return -ENXIO immediately when the target cpu is going down, that
means, we can load the vmcs but it is not cleared on target cpu and corrupt
per_cpu(loaded_vmcss_on_cpu, cpu). This bug can be triggered under this case:

# general protection fault: 0000 [#1] PREEMPT SMP


Call Trace:
[<ffffffffa052980f>] kvm_arch_hardware_disable+0x1f/0x50 [kvm]
[<ffffffffa050ef43>] hardware_disable_nolock+0x33/0x40 [kvm]
[<ffffffffa050efa3>] kvm_cpu_hotplug+0x53/0xb0 [kvm]
[<ffffffff81548b1d>] notifier_call_chain+0x4d/0x70
[<ffffffff81517fe0>] ? spp_getpage+0xb0/0xb0
[<ffffffff8108459e>] __raw_notifier_call_chain+0xe/0x10
[<ffffffff810599f0>] __cpu_notify+0x20/0x40
[<ffffffff8151802e>] take_cpu_down+0x4e/0x90
[<ffffffff810d184b>] cpu_stopper_thread+0xdb/0x1d0
[<ffffffff8108b3ce>] ? finish_task_switch+0x4e/0xe0
[<ffffffff815438d0>] ? __schedule+0x460/0x740
[<ffffffff810d1770>] ? cpu_stop_signal_done+0x40/0x40
[<ffffffff8107de30>] kthread+0xc0/0xd0
[<ffffffff8107dd70>] ? flush_kthread_worker+0xb0/0xb0
[<ffffffff8154cc6c>] ret_from_fork+0x7c/0xb0
[<ffffffff8107dd70>] ? flush_kthread_worker+0xb0/0xb0

Fix it by waiting for target vcpu to clear the vmcs

Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>

arch/x86/kvm/vmx.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 95e502b..4fb4e51 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1019,8 +1019,15 @@ static void loaded_vmcs_clear(struct loaded_vmcs *loaded_vmcs)
int cpu = loaded_vmcs->cpu;

if (cpu != -1)
- smp_call_function_single(cpu,
- __loaded_vmcs_clear, loaded_vmcs, 1);
+ if (smp_call_function_single(cpu,
+ __loaded_vmcs_clear, loaded_vmcs, 1))
+ /*
+ * The target cpu is going down, we should
+ * wait for it to clear the vmcs status.
+ */
+ while (ACCESS_ONCE(loaded_vmcs->cpu) != -1)
+ cpu_relax();

static inline void vpid_sync_vcpu_single(struct vcpu_vmx *vmx)

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