Re: [PATCH 2/2] KVM: VMX: fix memory order between loading vmcs andclearing vmcs

From: Xiao Guangrong
Date: Wed Nov 28 2012 - 22:07:18 EST


On 11/29/2012 08:04 AM, Marcelo Tosatti wrote:
> On Wed, Nov 28, 2012 at 08:54:14PM +0800, Xiao Guangrong wrote:
>> vmcs->cpu indicates whether it exists on the target cpu, -1 means the vmcs
>> does not exist on any vcpu
>>
>> If vcpu load vmcs with vmcs.cpu = -1, it can be directly added to cpu's percpu
>> list. The list can be corrupted if the cpu prefetch the vmcs's list before
>> reading vmcs->cpu. Meanwhile, we should remove vmcs from the list before
>> making vmcs->vcpu == -1 be visible
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
>> ---
>> arch/x86/kvm/vmx.c | 17 +++++++++++++++++
>> 1 files changed, 17 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 29e8f42..6056d88 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -1002,6 +1002,15 @@ static void __loaded_vmcs_clear(void *arg)
>> if (per_cpu(current_vmcs, cpu) == loaded_vmcs->vmcs)
>> per_cpu(current_vmcs, cpu) = NULL;
>> list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
>> +
>> + /*
>> + * we should ensure updating loaded_vmcs->loaded_vmcss_on_cpu_link
>> + * is before setting loaded_vmcs->vcpu to -1 which is done in
>> + * loaded_vmcs_init. Otherwise, other cpu can see vcpu = -1 fist
>> + * then adds the vmcs into percpu list before it is deleted.
>> + */
>> + smp_wmb();
>> +
>
> Neither loads nor stores are reordered with like operations (see
> section 8.2.3.2 of intel's volume 3). This behaviour makes the barrier
> not necessary.

Ouch, yes, you are right. My memory is wrong. It seems only later-read
can be reordered with early-write.

But if 'read vs read' and 'write vs write' can be guaranteed by CPU, smp_wmb()
and smp_rmb() should act as a complier barrier, so i think we can add the barriers
to improve the readable and the portable.

And anyway, the current code missed complier-barrier.

>
> However, i agree access to loaded_vmcs is not obviously safe. I can't
> tell its safe with vmm_exclusive = 0 (where vcpu->cpu can change at any
> time).

If vmm_exclusive = 0, the vmcs can removed from percpu list when vcpu is scheduled
out. The list is not broken.



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