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

Avi Kivity
2007-07-26T16:31:52Z

Avi Kivity wrote:
>>
>> 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.

btw, I think the race is benign. if __vcpu_clear() wins, vcpu_load()
gets to set current_vmcs and all is well. If vcpu_load() wins,
__vcpu_clear() stomps on current_vmcs, but the only effect of that the
next time vcpu_load() is called, it issues an unnecessary vmptrld.