On Fri, Jul 16, 2021 at 1:54 AM Zhu Lingshan <lingshan.zhu@xxxxxxxxx> wrote: > > The guest Precise Event Based Sampling (PEBS) feature can provide an > architectural state of the instruction executed after the guest instruction > that exactly caused the event. It needs new hardware facility only available > on Intel Ice Lake Server platforms. This patch set enables the basic PEBS > feature for KVM guests on ICX. > > We can use PEBS feature on the Linux guest like native: > > # echo 0 > /proc/sys/kernel/watchdog (on the host) > # perf record -e instructions:ppp ./br_instr a > # perf record -c 100000 -e instructions:pp ./br_instr a > > To emulate guest PEBS facility for the above perf usages, > we need to implement 2 code paths: > > 1) Fast path > > This is when the host assigned physical PMC has an identical index as the > virtual PMC (e.g. using physical PMC0 to emulate virtual PMC0). > This path is used in most common use cases. > > 2) Slow path > > This is when the host assigned physical PMC has a different index from the > virtual PMC (e.g. using physical PMC1 to emulate virtual PMC0) In this case, > KVM needs to rewrite the PEBS records to change the applicable counter indexes > to the virtual PMC indexes, which would otherwise contain the physical counter > index written by PEBS facility, and switch the counter reset values to the > offset corresponding to the physical counter indexes in the DS data structure. > > The previous version [0] enables both fast path and slow path, which seems > a bit more complex as the first step. In this patchset, we want to start with > the fast path to get the basic guest PEBS enabled while keeping the slow path > disabled. More focused discussion on the slow path [1] is planned to be put to > another patchset in the next step. > > Compared to later versions in subsequent steps, the functionality to support > host-guest PEBS both enabled and the functionality to emulate guest PEBS when > the counter is cross-mapped are missing in this patch set > (neither of these are typical scenarios). I'm not sure exactly what scenarios you're ruling out here. In our environment, we always have to be able to support host-level profiling, whether or not the guest is using the PMU (for PEBS or anything else). Hence, for our *basic* vPMU offering, we only expose two general purpose counters to the guest, so that we can keep two general purpose counters for the host. In this scenario, I would expect cross-mapped counters to be common. Are we going to be able to use this implementation?
