Re: [PATCH 2/2] KVM: Protect race-condition between VMCS and current_vmcs on VMX hardware

Avi Kivity
2007-07-26T15:35:02Z

Gregory Haskins wrote:
> On Thu, 2007-07-26 at 18:03 +0300, Avi Kivity wrote:
>
>> Gregory Haskins wrote:
>>
>>> We need to provide locking around the current_vmcs/VMCS interactions to
>>> protect against race conditions.
>>>
>>>
>>>
>> Can you explain the race?
>>
>
> Sure. It can happen with two VMs are running simultaneously. Lets call
> them VM-a and VM-b. Assume the scenario: VM-a is on CPU-x, gets
> migrated to CPU-y, and VM-b gets scheduled in on CPU-x. There is a race
> on CPU-x with the VMCS handling logic between the VM-b process context,
> and the IPI to execute the __vcpu_clear for VM-a.
>
>

A race indeed, good catch.

I think the race is only on the per_cpu(current_vmcs) variable, no? The
actual vmcs ptr (as loaded by vmptrld) is handled by the processor.

> Disabling interrupts was chosen as the sync-primitive, because the code
> will always be on the CPU in question.
>
>

Looks a bit heavy handed. How about replacing (in __vcpu_clear())

if (per_cpu(current_vmcs, cpu) == vcpu->vmcs)
per_cpu(current_vmcs, cpu) = NULL;

by

cmpxchg_local(&per_cpu(current_vmcs, cpu), vcpu->vmcs, NULL);

?