[PATCH] KVM: arm/arm64: Close VMID generation race

[Shannon Zhao]

On 2018/4/10 23:37, Marc Zyngier wrote:
> On 10/04/18 16:24, Mark Rutland wrote:
>> On Tue, Apr 10, 2018 at 05:05:40PM +0200, Christoffer Dall wrote:
>>> On Tue, Apr 10, 2018 at 11:51:19AM +0100, Mark Rutland wrote:
>>>> I think we also need to update kvm->arch.vttbr before updating
>>>> kvm->arch.vmid_gen, otherwise another CPU can come in, see that the
>>>> vmid_gen is up-to-date, jump to hyp, and program a stale VTTBR (with the
>>>> old VMID).
>>>>
>>>> With the smp_wmb() and update of kvm->arch.vmid_gen moved to the end of
>>>> the critical section, I think that works, modulo using READ_ONCE() and
>>>> WRITE_ONCE() to ensure single-copy-atomicity of the fields we access
>>>> locklessly.
>>>
>>> Indeed, you're right.  I would look something like this, then:
>>>
>>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>>> index 2e43f9d42bd5..6cb08995e7ff 100644
>>> --- a/virt/kvm/arm/arm.c
>>> +++ b/virt/kvm/arm/arm.c
>>> @@ -450,7 +450,9 @@ void force_vm_exit(const cpumask_t *mask)
>>>   */
>>>  static bool need_new_vmid_gen(struct kvm *kvm)
>>>  {
>>> -	return unlikely(kvm->arch.vmid_gen != atomic64_read(&kvm_vmid_gen));
>>> +	u64 current_vmid_gen = atomic64_read(&kvm_vmid_gen);
>>> +	smp_rmb(); /* Orders read of kvm_vmid_gen and kvm->arch.vmid */
>>> +	return unlikely(READ_ONCE(kvm->arch.vmid_gen) != current_vmid_gen);
>>>  }
>>>  
>>>  /**
>>> @@ -500,7 +502,6 @@ static void update_vttbr(struct kvm *kvm)
>>>  		kvm_call_hyp(__kvm_flush_vm_context);
>>>  	}
>>>  
>>> -	kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen);
>>>  	kvm->arch.vmid = kvm_next_vmid;
>>>  	kvm_next_vmid++;
>>>  	kvm_next_vmid &= (1 << kvm_vmid_bits) - 1;
>>> @@ -509,7 +510,10 @@ static void update_vttbr(struct kvm *kvm)
>>>  	pgd_phys = virt_to_phys(kvm->arch.pgd);
>>>  	BUG_ON(pgd_phys & ~VTTBR_BADDR_MASK);
>>>  	vmid = ((u64)(kvm->arch.vmid) << VTTBR_VMID_SHIFT) & VTTBR_VMID_MASK(kvm_vmid_bits);
>>> -	kvm->arch.vttbr = pgd_phys | vmid;
>>> +	WRITE_ONCE(kvm->arch.vttbr, pgd_phys | vmid);
>>> +
>>> +	smp_wmb(); /* Ensure vttbr update is observed before vmid_gen update */
>>> +	kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen);
>>>  
>>>  	spin_unlock(&kvm_vmid_lock);
>>>  }
>>
>> I think that's right, yes.
>>
>> We could replace the smp_{r,w}mb() barriers with an acquire of the
>> kvm_vmid_gen and a release of kvm->arch.vmid_gen, but if we're really
>> trying to optimize things there are larger algorithmic changes necessary
>> anyhow.
>>
>>> It's probably easier to convince ourselves about the correctness of
>>> Marc's code using a rwlock instead, though.  Thoughts?
>>
>> I believe that Marc's preference was the rwlock; I have no preference
>> either way.
> 
> I don't mind either way. If you can be bothered to write a proper commit
> log for this, I'll take it. What I'd really want is Shannon to indicate
> whether or not this solves the issue he was seeing.
> 
I'll test Marc's patch. This will take about 3 days since it's not 100%
reproducible.

Thanks,
-- 
Shannon