On Thu, Nov 27, 2014 at 4:10 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > On 27/11/14 10:22, Anup Patel wrote: >> On Tue, Nov 25, 2014 at 7:12 PM, Christoffer Dall >> <christoffer.dall@xxxxxxxxxx> 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 >>>> <christoffer.dall@xxxxxxxxxx> 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 >>>>>> <christoffer.dall@xxxxxxxxxx> 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 >>>>>>>> <christoffer.dall@xxxxxxxxxx> 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 >>>>>>>>>> <christoffer.dall@xxxxxxxxxx> 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? Currently, irqd_set_irq_forwarded() is lockless. It would be great if we can update irqd_set_irq_forwarded() for PPIs such that it remains irqd_set_irq_forwarded() lockless so that we dont have much overhead when we enable/disable forwarding state. > > Thanks, > > M. > -- > Jazz is not dead. It just smells funny... Regards, Anup -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
