Re: [PATCH 1/2] KVM: arm64: Add PMU event filtering infrastructure

[Robin Murphy <robin.murphy@arm.com>]

On 15/02/2020 10:28 am, Marc Zyngier wrote:
> On Fri, 14 Feb 2020 22:01:01 +0000,
> Robin Murphy <robin.murphy@arm.com> wrote:
> 
> Hi Robin,
> 
>>
>> Hi Marc,
>>
>> On 2020-02-14 6:36 pm, Marc Zyngier wrote:
>> [...]
>>> @@ -585,6 +585,14 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
>>>    	    pmc->idx != ARMV8_PMU_CYCLE_IDX)
>>>    		return;
>>>    +	/*
>>> +	 * If we have a filter in place and that the event isn't allowed, do
>>> +	 * not install a perf event either.
>>> +	 */
>>> +	if (vcpu->kvm->arch.pmu_filter &&
>>> +	    !test_bit(eventsel, vcpu->kvm->arch.pmu_filter))
>>> +		return;
>>
>> If I'm reading the derivation of eventsel right, this will end up
>> treating cycle counter events (aliased to SW_INCR) differently from
>> CPU_CYCLES, which doesn't seem desirable.
> 
> Indeed, this doesn't look quite right.
> 
> Looking at the description of event 0x11, it doesn't seem to count
> exactly like the cycle counter (there are a number of PMCR controls
> affecting it). But none of these actually apply to our PMU emulation
> (no secure mode, and the idea of dealing with virtual EL2 in the
> context of the PMU is... not appealing).
> 
> Now, given that we implement the cycle counter with event 0x11 anyway,
> I don't think there is any reason to deal with them separately.

Right, from the user's PoV they can only ask for event 0x11, and where 
it gets scheduled is more of a black-box implementation detail. Reading 
the Arm ARM doesn't leave me entirely convinced that cycles couldn't 
ever leak idle/not-idle information between closely-coupled PEs, so this 
might not be entirely academic.

>> Also, if the user did try to blacklist SW_INCR for ridiculous
>> reasons, we'd need to special-case kvm_pmu_software_increment() to
>> make it (not) work as expected, right?
> 
> I thought of that one, and couldn't see a reason to blacklist it
> (after all, the guest could also increment a variable) and send itself
> an interrupt. I'm tempted to simply document that event 0 is never
> filtered.

I'd say you're on even stronger ground simply because KVM's 
implementation of SW_INCR doesn't go near the PMU hardware at all, thus 
is well beyond the purpose of the blacklist anyway. I believe it's 
important that how the code behaves matches expectations, but there's no 
harm in changing the latter as appropriate ;)

Cheers,
Robin.
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