On 15/02/2020 10:28 am, Marc Zyngier wrote:
On Fri, 14 Feb 2020 22:01:01 +0000,
Robin Murphy <robin.murphy@xxxxxxx> 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.
