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

[Anup Patel <anup@brainfault.org>]

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?

If not then can we consider current mask/unmask approach
for forwarding PMU IRQs?

Marc?? Will??

Regards,
Anup