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. What about the scenario1 which I had mentioned? -- Anup > > -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
