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

[Christoffer Dall]

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 at linaro.org> 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 at linaro.org> 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 at linaro.org> 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.

Thanks,
-Christoffer