Re: [PATCH V9 00/18] KVM: x86/pmu: Add *basic* support to enable guest PEBS via DS

[Like Xu <like.xu.linux@gmail.com>]

On Wed, Jul 28, 2021 at 11:46 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jul 22, 2021 at 01:41:41PM +0800, Zhu Lingshan 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
>
> Why does the host need to disable the watchdog? IIRC ICL has multiple
> PEBS capable counters. Also, I think the watchdog ends up on a fixed
> counter by default anyway.

The watchdog counter blocks the KVM PEBS request on the same (fixed) counter.
This restriction will be lifted when we have cross-mapping support later in KVM.

>
> > Like Xu (17):
> >   perf/core: Use static_call to optimize perf_guest_info_callbacks
> >   perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server
> >   perf/x86/intel: Handle guest PEBS overflow PMI for KVM guest
> >   perf/x86/core: Pass "struct kvm_pmu *" to determine the guest values
> >   KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit when vPMU is enabled
> >   KVM: x86/pmu: Introduce the ctrl_mask value for fixed counter
> >   KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS
> >   KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter
> >   KVM: x86/pmu: Adjust precise_ip to emulate Ice Lake guest PDIR counter
> >   KVM: x86/pmu: Add IA32_DS_AREA MSR emulation to support guest DS
> >   KVM: x86/pmu: Add PEBS_DATA_CFG MSR emulation to support adaptive PEBS
> >   KVM: x86: Set PEBS_UNAVAIL in IA32_MISC_ENABLE when PEBS is enabled
> >   KVM: x86/pmu: Move pmc_speculative_in_use() to arch/x86/kvm/pmu.h
> >   KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations
> >   KVM: x86/pmu: Add kvm_pmu_cap to optimize perf_get_x86_pmu_capability
> >   KVM: x86/cpuid: Refactor host/guest CPU model consistency check
> >   KVM: x86/pmu: Expose CPUIDs feature bits PDCM, DS, DTES64
> >
> > Peter Zijlstra (Intel) (1):
> >   x86/perf/core: Add pebs_capable to store valid PEBS_COUNTER_MASK value
>
> Looks good:
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Thanks for your time and support of the guest PMU features.

> How do we want to route this, all through the KVM tree?

As a prerequisite, the perf tree may apply the first three patches.
Hi Paolo, do you have any preferences ?

>
> One little nit I had; would something like the below (on top perhaps)
> make the code easier to read?

Fine to me and I may provide a follow-up patch.

>
> ---
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -3921,9 +3921,12 @@ static struct perf_guest_switch_msr *int
>         struct kvm_pmu *kvm_pmu = (struct kvm_pmu *)data;
>         u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
>         u64 pebs_mask = cpuc->pebs_enabled & x86_pmu.pebs_capable;
> +       int global_ctrl, pebs_enable;
>
>         *nr = 0;
> -       arr[(*nr)++] = (struct perf_guest_switch_msr){
> +
> +       global_ctrl = (*nr)++;
> +       arr[global_ctrl] = (struct perf_guest_switch_msr){
>                 .msr = MSR_CORE_PERF_GLOBAL_CTRL,
>                 .host = intel_ctrl & ~cpuc->intel_ctrl_guest_mask,
>                 .guest = intel_ctrl & (~cpuc->intel_ctrl_host_mask | ~pebs_mask),
> @@ -3966,23 +3969,23 @@ static struct perf_guest_switch_msr *int
>                 };
>         }
>
> -       arr[*nr] = (struct perf_guest_switch_msr){
> +       pebs_enable = (*nr)++;
> +       arr[pebs_enable] = (struct perf_guest_switch_msr){
>                 .msr = MSR_IA32_PEBS_ENABLE,
>                 .host = cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask,
>                 .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
>         };
>
> -       if (arr[*nr].host) {
> +       if (arr[pebs_enable].host) {
>                 /* Disable guest PEBS if host PEBS is enabled. */
> -               arr[*nr].guest = 0;
> +               arr[pebs_enable].guest = 0;
>         } else {
>                 /* Disable guest PEBS for cross-mapped PEBS counters. */
> -               arr[*nr].guest &= ~kvm_pmu->host_cross_mapped_mask;
> +               arr[pebs_enable].guest &= ~kvm_pmu->host_cross_mapped_mask;
>                 /* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
> -               arr[0].guest |= arr[*nr].guest;
> +               arr[global_ctrl].guest |= arr[pebs_enable].guest;
>         }
>
> -       ++(*nr);
>         return arr;
>  }
>
>
>
>