Subject: [RFC PATCH 0/6] ARM64: KVM: PMU infrastructure support

From: [email-masked] (Marc Zyngier)
Date: Thu, 27 Nov 2014 10:40:47 +0000

On 27/11/14 10:22, Anup Patel wrote:
> On Tue, Nov 25, 2014 at 7:12 PM, Christoffer Dall
> <[email-masked]> wrote:
>> On Tue, Nov 25, 2014 at 06:17:03PM +0530, Anup Patel wrote:
>>> Hi Christoffer,
>>>
>>> On Mon, Nov 24, 2014 at 8:07 PM, Christoffer Dall
>>> <[email-masked]> wrote:
>>>> On Mon, Nov 24, 2014 at 02:14:48PM +0530, Anup Patel wrote:
>>>>> On Fri, Nov 21, 2014 at 5:19 PM, Christoffer Dall
>>>>> <[email-masked]> wrote:
>>>>>> On Fri, Nov 21, 2014 at 04:06:05PM +0530, Anup Patel wrote:
>>>>>>> Hi Christoffer,
>>>>>>>
>>>>>>> On Fri, Nov 21, 2014 at 3:29 PM, Christoffer Dall
>>>>>>> <[email-masked]> wrote:
>>>>>>>> On Thu, Nov 20, 2014 at 08:17:32PM +0530, Anup Patel wrote:
>>>>>>>>> On Wed, Nov 19, 2014 at 8:59 PM, Christoffer Dall
>>>>>>>>> <[email-masked]> wrote:
>>>>>>>>>> On Tue, Nov 11, 2014 at 02:48:25PM +0530, Anup Patel wrote:
>>>>>>>>>>> Hi All,
>>>>>>>>>>>
>>>>>>>>>>> I have second thoughts about rebasing KVM PMU patches
>>>>>>>>>>> to Marc's irq-forwarding patches.
>>>>>>>>>>>
>>>>>>>>>>> The PMU IRQs (when virtualized by KVM) are not exactly
>>>>>>>>>>> forwarded IRQs because they are shared between Host
>>>>>>>>>>> and Guest.
>>>>>>>>>>>
>>>>>>>>>>> Scenario1
>>>>>>>>>>> -------------
>>>>>>>>>>>
>>>>>>>>>>> We might have perf running on Host and no KVM guest
>>>>>>>>>>> running. In this scenario, we wont get interrupts on Host
>>>>>>>>>>> because the kvm_pmu_hyp_init() (similar to the function
>>>>>>>>>>> kvm_timer_hyp_init() of Marc's IRQ-forwarding
>>>>>>>>>>> implementation) has put all host PMU IRQs in forwarding
>>>>>>>>>>> mode.
>>>>>>>>>>>
>>>>>>>>>>> The only way solve this problem is to not set forwarding
>>>>>>>>>>> mode for PMU IRQs in kvm_pmu_hyp_init() and instead
>>>>>>>>>>> have special routines to turn on and turn off the forwarding
>>>>>>>>>>> mode of PMU IRQs. These routines will be called from
>>>>>>>>>>> kvm_arch_vcpu_ioctl_run() for toggling the PMU IRQ
>>>>>>>>>>> forwarding state.
>>>>>>>>>>>
>>>>>>>>>>> Scenario2
>>>>>>>>>>> -------------
>>>>>>>>>>>
>>>>>>>>>>> We might have perf running on Host and Guest simultaneously
>>>>>>>>>>> which means it is quite likely that PMU HW trigger IRQ meant
>>>>>>>>>>> for Host between "ret = kvm_call_hyp(__kvm_vcpu_run, vcpu);"
>>>>>>>>>>> and "kvm_pmu_sync_hwstate(vcpu);" (similar to timer sync routine
>>>>>>>>>>> of Marc's patchset which is called before local_irq_enable()).
>>>>>>>>>>>
>>>>>>>>>>> In this scenario, the updated kvm_pmu_sync_hwstate(vcpu)
>>>>>>>>>>> will accidentally forward IRQ meant for Host to Guest unless
>>>>>>>>>>> we put additional checks to inspect VCPU PMU state.
>>>>>>>>>>>
>>>>>>>>>>> Am I missing any detail about IRQ forwarding for above
>>>>>>>>>>> scenarios?
>>>>>>>>>>>
>>>>>>>>>> Hi Anup,
>>>>>>>>>
>>>>>>>>> Hi Christoffer,
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I briefly discussed this with Marc.  What I don't understand is how it
>>>>>>>>>> would be possible to get an interrupt for the host while running the
>>>>>>>>>> guest?
>>>>>>>>>>
>>>>>>>>>> The rationale behind my question is that whenever you're running the
>>>>>>>>>> guest, the PMU should be programmed exclusively with guest state, and
>>>>>>>>>> since the PMU is per core, any interrupts should be for the guest, where
>>>>>>>>>> it would always be pending.
>>>>>>>>>
>>>>>>>>> Yes, thats right PMU is programmed exclusively for guest when
>>>>>>>>> guest is running and for host when host is running.
>>>>>>>>>
>>>>>>>>> Let us assume a situation (Scenario2 mentioned previously)
>>>>>>>>> where both host and guest are using PMU. When the guest is
>>>>>>>>> running we come back to host mode due to variety of reasons
>>>>>>>>> (stage2 fault, guest IO, regular host interrupt, host interrupt
>>>>>>>>> meant for guest, ....) which means we will return from the
>>>>>>>>> "ret = kvm_call_hyp(__kvm_vcpu_run, vcpu);" statement in the
>>>>>>>>> kvm_arch_vcpu_ioctl_run() function with local IRQs disabled.
>>>>>>>>> At this point we would have restored back host PMU context and
>>>>>>>>> any PMU counter used by host can trigger PMU overflow interrup
>>>>>>>>> for host. Now we will be having "kvm_pmu_sync_hwstate(vcpu);"
>>>>>>>>> in the kvm_arch_vcpu_ioctl_run() function (similar to the
>>>>>>>>> kvm_timer_sync_hwstate() of Marc's IRQ forwarding patchset)
>>>>>>>>> which will try to detect PMU irq forwarding state in GIC hence it
>>>>>>>>> can accidentally discover PMU irq pending for guest while this
>>>>>>>>> PMU irq is actually meant for host.
>>>>>>>>>
>>>>>>>>> This above mentioned situation does not happen for timer
>>>>>>>>> because virtual timer interrupts are exclusively used for guest.
>>>>>>>>> The exclusive use of virtual timer interrupt for guest ensures that
>>>>>>>>> the function kvm_timer_sync_hwstate() will always see correct
>>>>>>>>> state of virtual timer IRQ from GIC.
>>>>>>>>>
>>>>>>>> I'm not quite following.
>>>>>>>>
>>>>>>>> When you call kvm_pmu_sync_hwstate(vcpu) in the non-preemtible section,
>>>>>>>> you would (1) capture the active state of the IRQ pertaining to the
>>>>>>>> guest and (2) deactive the IRQ on the host, then (3) switch the state of
>>>>>>>> the PMU to the host state, and finally (4) re-enable IRQs on the CPU
>>>>>>>> you're running on.
>>>>>>>>
>>>>>>>> If the host PMU state restored in (3) causes the PMU to raise an
>>>>>>>> interrupt, you'll take an interrupt after (4), which is for the host,
>>>>>>>> and you'll handle it on the host.
>>>>>>>>
>>>>>>> We only switch PMU state in assembly code using
>>>>>>> kvm_call_hyp(__kvm_vcpu_run, vcpu)
>>>>>>> so whenever we are in kvm_arch_vcpu_ioctl_run() (i.e. host mode)
>>>>>>> the current hardware PMU state is for host. This means whenever
>>>>>>> we are in host mode the host PMU can change state of PMU IRQ
>>>>>>> in GIC even if local IRQs are disabled.
>>>>>>>
>>>>>>> Whenever we inspect active state of PMU IRQ in the
>>>>>>> kvm_pmu_sync_hwstate() function using irq_get_fwd_state() API.
>>>>>>> Here we are not guaranteed that IRQ forward state returned by the
>>>>>>> irq_get_fwd_state() API is for guest only.
>>>>>>>
>>>>>>> The above situation does not manifest for virtual timer because
>>>>>>> virtual timer registers are exclusively accessed by Guest and
>>>>>>> virtual timer interrupt is only for Guest (never used by Host).
>>>>>>>
>>>>>>>> Whenever you schedule the guest VCPU again, you'll (a) disable
>>>>>>>> interrupts on the CPU, (b) restore the active state of the IRQ for the
>>>>>>>> guest, (c) restore the guest PMU state, (d) switch to the guest with
>>>>>>>> IRQs enabled on the CPU (potentially).
>>>>>>>
>>>>>>> Here too, while we are between step (a) and step (b) the PMU HW
>>>>>>> context is for host and any PMU counter can overflow. The step (b)
>>>>>>> can actually override the PMU IRQ meant for Host.
>>>>>>>
>>>>>> Can you not simply switch the state from C-code after capturing the IRQ
>>>>>> state then?  Everything should be accessible from EL1, right?
>>>>>
>>>>> Yes, I think that would be the only option. This also means I will need
>>>>> to re-implement context switching for doing it in C-code.
>>>>
>>>> Yes, you'd add some inline assembly in the C-code to access the
>>>> registers I guess.  Only thing I thought about after writing my original
>>>> mail is whether you'll be counting events while context-swtiching and
>>>> running on the host, which you actually don't want to.  Not sure if
>>>> there's a better way to avoid that.
>>>>
>>>>>
>>>>> What about the scenario1 which I had mentioned?
>>>>>
>>>>
>>>> You have to consider enabling/disabling forwarding and setting/clearing
>>>> the active state is part of the guest PMU state and all of it has to be
>>>> context-switched.
>>>
>>> I found one more issue.
>>>
>>> If PMU irq is PPI then enabling/disabling forwarding will not
>>> work because irqd_set_irq_forwarded() function takes irq_data
>>> as argument which is member of irq_desc and irq_desc for PPIs
>>> is not per_cpu. This means we cannot call irqd_set_irq_forwarded()
>>> simultaneously from different host CPUs.
>>>
> 
> Hi Marc,
> 
>> I'll let Marc answer this one and if this still applies to his view of
>> how the next version of the forwarding series will look like.

I'm looking at it at the moment.

I'm inclined to say that we should fix the forwarding code to allow
individual PPIs to be forwarded. This is a bit harder than what we're
doing at the moment, but that's possible.

Of course, that complicates the code a bit, as we have to make sure
we're not premptable at that time.

What do you think?

Thanks,

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