On Tue, Jul 08, 2025 at 10:38:35PM +0000, Colton Lewis wrote: > Oliver Upton <oliver.upton@xxxxxxxxx> writes: > > > On Mon, Jul 07, 2025 at 05:57:14PM +0100, Mark Rutland wrote: > > > On Thu, Jun 26, 2025 at 08:04:42PM +0000, Colton Lewis wrote: > > > > For PMUv3, the register field MDCR_EL2.HPMN partitiones the PMU > > > > counters into two ranges where counters 0..HPMN-1 are accessible by > > > > EL1 and, if allowed, EL0 while counters HPMN..N are only accessible by > > > > EL2. > > > > > > > > Create module parameter reserved_host_counters to reserve a number of > > > > counters for the host. This number is set at boot because the perf > > > > subsystem assumes the number of counters will not change after the PMU > > > > is probed. > > > > > > > > Introduce the function armv8pmu_partition() to modify the PMU driver's > > > > cntr_mask of available counters to exclude the counters being reserved > > > > for the guest and record reserved_guest_counters as the maximum > > > > allowable value for HPMN. > > > > > > > > Due to the difficulty this feature would create for the driver running > > > > at EL1 on the host, partitioning is only allowed in VHE mode. Working > > > > on nVHE mode would require a hypercall for every counter access in the > > > > driver because the counters reserved for the host by HPMN are only > > > > accessible to EL2. > > > > It would be good if we could elaborate on this last point. When exactly > > > do we intend to configure HPMN (e.g. is that static, dynamic at > > > load/put, or dynamic at finer granularity)? > > > > I ask becuase it's not immediately clear to me how this would break nVHE > > > without also breaking direct userspace access on VHE, unless we flip > > > HPMN dynamically at load/put, and this is only broken in some transient > > > windows on nVHE. > > > Agree that KVM's HPMN can only take effect between vcpu_load() / > > vcpu_put(). > > > The changelog isn't correct regarding the complications of nVHE, though. > > In order to support a 'partitioned' PMU on nVHE we'd need to explicitly > > disable guest counters on every exit and reset HPMN to place all > > counters in the 'first range'. Unless someone has a use case for this > > stuff on nVHE I'm not too bothered by the VHE-only limitation. > > I'll fix this. > > > > > > > > > Signed-off-by: Colton Lewis <coltonlewis@xxxxxxxxxx> > > > > --- > > > > arch/arm/include/asm/arm_pmuv3.h | 14 ++++++ > > > > arch/arm64/include/asm/arm_pmuv3.h | 5 ++ > > > > arch/arm64/include/asm/kvm_pmu.h | 6 +++ > > > > arch/arm64/kvm/Makefile | 2 +- > > > > arch/arm64/kvm/pmu-part.c | 23 ++++++++++ > > > > Maybe I'll contradict Oliver and Marc here (and whatever they say > > > rules), but IMO it'd be nice to spell out "partition" rather than "part" > > > here for clarity. > > > I'm not too big of a fan of the naming here either. I'd prefer something > > like "pmu-direct". Partitioning is just a side effect of how we're > > allocating counters currently and most of this implementation could be > > reused if we pass the entire PMU to the guest in the future. > > Sure. > > > With that being said -- Colton I'd focus on getting these patches in > > shape while we figure out what color we want it ;-) > > > Thanks, > > Oliver > > Trust me I'm working on it. I know you are -- this comment wasn't meant to add pressure to you. Just noting that we might bikeshed a bit further on the naming here but I wouldn't worry much about it for the time being. sed isn't _that_ difficult :) Thanks, Oliver
