Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if the interrupt is not single-destination

[Yang Zhang <yang.zhang.wz@gmail.com>]

On 2016/1/21 13:07, Wu, Feng wrote:
>
>
>> -----Original Message-----
>> From: Yang Zhang [mailto:yang.zhang.wz@gmail.com]
>> Sent: Thursday, January 21, 2016 1:00 PM
>> To: Wu, Feng <feng.wu@intel.com>; pbonzini@redhat.com;
>> rkrcmar@redhat.com
>> Cc: linux-kernel@vger.kernel.org; kvm@vger.kernel.org
>> Subject: Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if the
>> interrupt is not single-destination
>>
>> On 2016/1/21 12:42, Wu, Feng wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: kvm-owner@vger.kernel.org [mailto:kvm-owner@vger.kernel.org]
>> On
>>>> Behalf Of Yang Zhang
>>>> Sent: Thursday, January 21, 2016 11:35 AM
>>>> To: Wu, Feng <feng.wu@intel.com>; pbonzini@redhat.com;
>>>> rkrcmar@redhat.com
>>>> Cc: linux-kernel@vger.kernel.org; kvm@vger.kernel.org
>>>> Subject: Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if the
>>>> interrupt is not single-destination
>>>>
>>>> On 2016/1/21 11:14, Wu, Feng wrote:
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Yang Zhang [mailto:yang.zhang.wz@gmail.com]
>>>>>> Sent: Thursday, January 21, 2016 11:06 AM
>>>>>> To: Wu, Feng <feng.wu@intel.com>; pbonzini@redhat.com;
>>>>>> rkrcmar@redhat.com
>>>>>> Cc: linux-kernel@vger.kernel.org; kvm@vger.kernel.org
>>>>>> Subject: Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if
>> the
>>>>>> interrupt is not single-destination
>>>>>>
>>>>>> On 2016/1/20 9:42, Feng Wu wrote:
>>>>>>> When the interrupt is not single destination any more, we need
>>>>>>> to change back IRTE to remapped mode explicitly.
>>>>>>>
>>>>>>> Signed-off-by: Feng Wu <feng.wu@intel.com>
>>>>>>> ---
>>>>>>>      arch/x86/kvm/vmx.c | 11 ++++++++++-
>>>>>>>      1 file changed, 10 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>>>>> index e2951b6..13d14d4 100644
>>>>>>> --- a/arch/x86/kvm/vmx.c
>>>>>>> +++ b/arch/x86/kvm/vmx.c
>>>>>>> @@ -10764,8 +10764,17 @@ static int vmx_update_pi_irte(struct kvm
>>>>>> *kvm, unsigned int host_irq,
>>>>>>>      		 */
>>>>>>>
>>>>>>>      		kvm_set_msi_irq(e, &irq);
>>>>>>> -		if (!kvm_intr_is_single_vcpu(kvm, &irq, &vcpu))
>>>>>>> +		if (!kvm_intr_is_single_vcpu(kvm, &irq, &vcpu)) {
>>>>>>> +			/*
>>>>>>> +			 * Make sure the IRTE is in remapped mode if
>>>>>>> +			 * we don't handle it in posted mode.
>>>>>>> +			 */
>>>>>>> +			pi_set_sn(vcpu_to_pi_desc(vcpu));
>>>>>>> +			ret = irq_set_vcpu_affinity(host_irq, NULL);
>>>>>>> +			pi_clear_sn(vcpu_to_pi_desc(vcpu));
>>>>>>> +
>>>>>>>      			continue;
>>>>>>> +		}
>>>>>>>
>>>>>>>      		vcpu_info.pi_desc_addr =
>> __pa(vcpu_to_pi_desc(vcpu));
>>>>>>>      		vcpu_info.vector = irq.vector;
>>>>>>>
>>>>>>
>>>>>> I am still feel weird with this change: according the semantic of VT-d
>>>>>> posted interrupt, the interrupt will injected to guest through posted
>>>>>> notification and /proc/interrupts shows the same meaning. But now,
>>>>>> without being aware of user, the interrupt changes to legacy way and it
>>>>>> appears on different entry on /proc/interrupts. It looks weird.
>>>>>
>>>>> I don't think it has problem here, IMO, this is exactly how it works.
>>>>> There should be different entry for the interrupts in VT-d PI mode
>>>>> and leagcy mode.
>>>>
>>>> I am not saying any problem here. Just feel weird. From a normal user's
>>>> point, he has turned on the VT-d pi and according the semantic of VT-d
>>>> pi, he should not observe the interrupt through legacy mode, but now he
>>>> do see it. Maybe print out a message here will be helpful, like what you
>>>> did for disabled lapic found during irq injection.
>>>
>>> Even VT-d PI is on, not all interrupts can be handled by it, the reason the
>>
>> No, we can handle it but we don't do it due to the complexity.For
>> example, we can use wake up vector to delivery the interrupt which still
>> is in PI mode but doesn't require any mode change.
>
> I mean, multi-cast and broadcast interrupts cannot be handled in PI mode.

We may have different understanding on PI mode. My understanding is if 
we set the IRTE to PI format, than the subsequent interrupt will be 
handled in PI mode. multi-cast and broadcast interrupts cannot be 
injected to guest directly but it doesn't mean cannot be handled in PI 
mode. As i said, we can handle it in wake up vector or via other 
approach.But it is much complexity.

I agree that rollback to legacy mode is the best choice, but may need 
some additional messages to tell the user(host administrator) why we 
change to legacy mode. I think not all of them are familiar with the 
detail of VT-d PI. If they find there are still some interrupts goto 
legacy mode even they have turned on PI, they may get confused.

>
>>
>>> interrupts is changed back to legacy mode is because the user changes
>>> the affinity, and it cannot be handle in PI mode, and hence legacy mode
>>> is used. It is the user's behavior that cause this mode change, seems it is
>>> not so weird to me. But add some message here is good idea, just like
>>
>> Why user's behavior can change the mode?
>
> Like you mentioned before, if the interrupt is changed from single-destination
> to multiple-destination by guest. And this is the reason of adding the rollback
> logic here, right?

The user means the host administrator.

>
> Thanks,
> Feng
>
>> According the current design,
>> there is no way for user to turn on/off dynamically.Why we need to
>> rollback to legacy mode is we don't want to handle multi-destination
>> interrupt in PI mode but it doesn't mean we cannot do it like i said before.
>>
>>
>> --
>> best regards
>> yang


-- 
best regards
yang