Re: [PATCH] KVM: x86/pmu: Distinguish EVENTSEL bitmasks for uniform event creation and filtering

[Jim Mattson <jmattson@google.com>]

On Thu, Feb 10, 2022 at 2:26 AM Like Xu <like.xu.linux@gmail.com> wrote:
>
> From: Like Xu <likexu@tencent.com>
>
> The current usage of EVENTSEL_* macro is a mess in the KVM context. Partly
> because we have a conceptual ambiguity when choosing to create a RAW or
> HARDWARE event: when bits other than HARDWARE_EVENT_MASK are set,
> the pmc_reprogram_counter() will use the RAW type.
>
> By introducing the new macro AMD64_EXTRA_EVENTSEL_EVENT to simplify,
> the following three issues can be addressed in one go:
>
> - the 12 selection bits are used as comparison keys for allow or deny;
> - NON_HARDWARE_EVENT_MASK is only used to determine if a HARDWARE
>   event is programmed or not, a 12-bit selected event will be a RAW event;
>   (jmattson helped report this issue)
> - by reusing AMD64_RAW_EVENT_MASK, the extra 4 selection bits (if set) are
>   passed to the perf correctly and not filtered out by X86_RAW_EVENT_MASK;.
>
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
>  arch/x86/include/asm/perf_event.h |  3 ++-
>  arch/x86/kvm/pmu.c                | 11 ++++-------
>  arch/x86/kvm/pmu.h                |  6 ++++++
>  3 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> index 8fc1b5003713..bd068fd19043 100644
> --- a/arch/x86/include/asm/perf_event.h
> +++ b/arch/x86/include/asm/perf_event.h
> @@ -43,8 +43,9 @@
>  #define AMD64_EVENTSEL_INT_CORE_SEL_MASK               \
>         (0xFULL << AMD64_EVENTSEL_INT_CORE_SEL_SHIFT)
>
> +#define AMD64_EXTRA_EVENTSEL_EVENT                             (0x0FULL << 32)
>  #define AMD64_EVENTSEL_EVENT   \
> -       (ARCH_PERFMON_EVENTSEL_EVENT | (0x0FULL << 32))
> +       (ARCH_PERFMON_EVENTSEL_EVENT | AMD64_EXTRA_EVENTSEL_EVENT)
>  #define INTEL_ARCH_EVENT_MASK  \
>         (ARCH_PERFMON_EVENTSEL_UMASK | ARCH_PERFMON_EVENTSEL_EVENT)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 2c98f3ee8df4..99426a8d7f18 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -198,7 +198,8 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
>
>         filter = srcu_dereference(kvm->arch.pmu_event_filter, &kvm->srcu);
>         if (filter) {
> -               __u64 key = eventsel & AMD64_RAW_EVENT_MASK_NB;
> +               __u64 key = eventsel & (INTEL_ARCH_EVENT_MASK |
> +                                       AMD64_EXTRA_EVENTSEL_EVENT);
>
>                 if (bsearch(&key, filter->events, filter->nevents,
>                             sizeof(__u64), cmp_u64))
> @@ -209,18 +210,14 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
>         if (!allow_event)
>                 return;
>
> -       if (!(eventsel & (ARCH_PERFMON_EVENTSEL_EDGE |
> -                         ARCH_PERFMON_EVENTSEL_INV |
> -                         ARCH_PERFMON_EVENTSEL_CMASK |
> -                         HSW_IN_TX |
> -                         HSW_IN_TX_CHECKPOINTED))) {
> +       if (!(eventsel & NON_HARDWARE_EVENT_MASK)) {

I still don't understand why we even bother doing this lookup in the
first place. What's wrong with simply requesting PERF_TYPE_RAW every
time?

>                 config = kvm_x86_ops.pmu_ops->pmc_perf_hw_id(pmc);
>                 if (config != PERF_COUNT_HW_MAX)
>                         type = PERF_TYPE_HARDWARE;
>         }
>
>         if (type == PERF_TYPE_RAW)
> -               config = eventsel & X86_RAW_EVENT_MASK;
> +               config = eventsel & AMD64_RAW_EVENT_MASK;

This chunk looks a lot like
https://lore.kernel.org/kvm/20220203014813.2130559-2-jmattson@google.com/.
Note that if you don't increase the width of config (as in the first
change of that series), this mask change is ineffective.

>         if (pmc->current_config == eventsel && pmc_resume_counter(pmc))
>                 return;
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 7a7b8d5b775e..48d867e250bc 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -17,6 +17,12 @@
>
>  #define MAX_FIXED_COUNTERS     3
>
> +#define KVM_ARCH_PERFMON_EVENTSEL_IGNORE \
> +       (ARCH_PERFMON_EVENTSEL_ANY | ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
> +
> +#define NON_HARDWARE_EVENT_MASK        (AMD64_EXTRA_EVENTSEL_EVENT | \
> +        (X86_ALL_EVENT_FLAGS & ~KVM_ARCH_PERFMON_EVENTSEL_IGNORE))
> +
>  struct kvm_event_hw_type_mapping {
>         u8 eventsel;
>         u8 unit_mask;
> --
> 2.35.0
>