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.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
