Re: [PATCH V9 1/3] irq: Allow to pass the IRQF_TIMER flag with percpu irq request

[Daniel Lezcano]

On 25/04/2017 15:22, Marc Zyngier wrote:
> On 25/04/17 13:51, Daniel Lezcano wrote:
>> On Tue, Apr 25, 2017 at 11:21:21AM +0100, Marc Zyngier wrote:
>>> On 25/04/17 10:49, Daniel Lezcano wrote:
>>>> On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:
>>>
>>> [...]
>>>
>>>>>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
>>>>>> +{
>>>>>> +	/*
>>>>>> +	 * We don't need the measurement because the idle code already
>>>>>> +	 * knows the next expiry event.
>>>>>> +	 */
>>>>>> +	if (act->flags & __IRQF_TIMER)
>>>>>> +		return;
>>>>>
>>>>> And that's where this is really wrong for the KVM guest timer. As I
>>>>> said, this timer is under complete control of the guest, and the rest of
>>>>> the system doesn't know about it. KVM itself will only find out when the
>>>>> vcpu does a VM exit for a reason or another, and will just save/restore
>>>>> the state in order to be able to give the timer to another guest.
>>>>>
>>>>> The idle code is very much *not* aware of anything concerning that guest
>>>>> timer.
>>>>
>>>> Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1
>>>> at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.
>>>>
>>>> The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual
>>>> timer is under control of VM2 and will expire at <time+delta>.
>>>>
>>>> Is the host wake up with the SW timer and switch in VM1 which in turn restores
>>>> the timer and jump in the virtual timer irq handler?
>>>
>>> Indeed. The SW timer causes VM1 to wake-up, either on the same CPU
>>> (preempting VM2) or on another. The timer is then restored with the
>>> pending virtual interrupt injected, and the guest does what it has to
>>> with it.
>>
>> Thanks for clarification.
>>
>> So there is a virtual timer with real registers / interruption (waking up the
>> host) for the running VMs and SW timers for non-running VMs.
>>
>> What is the benefit of having such mechanism instead of real timers injecting
>> interrupts in the VM without the virtual timer + save/restore? Efficiency in
>> the running VMs when setting up timers (saving privileges change overhead)?
> 
> 
> You can't dedicate HW resources to virtual CPUs. It just doesn't scale.
> Also, injecting HW interrupts in a guest is pretty hard work, and for
> multiple reasons:
>   - the host needs to be in control of interrupt delivery (don't hog the
> CPU with guest interrupts)
>   - you want to be able to remap interrupts (id X on the host becomes id
> Y on the guest),
>   - you want to deal with migrating vcpus,
>   - you want deliver an interrupt to a vcpu that is *not* running.
> 
> It *is* doable, but it is not cheap at all from a HW point of view.


Ok, I see.

Thanks!

  -- Daniel


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