Re: [PATCH V8 00/18] KVM: x86/pmu: Add *basic* support to enable guest PEBS via DS

[Jim Mattson <jmattson@google.com>]

On Fri, Jul 16, 2021 at 1:54 AM Zhu Lingshan <lingshan.zhu@intel.com> 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?