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. > 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. -Christoffer -- 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
