Re: [PATCH v7] arm/kvm: Enable support for KVM_ARM_VCPU_PMU_V3_FILTER

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Hi Shaoqin,
On 2/21/24 07:34, Shaoqin Huang wrote:
> The KVM_ARM_VCPU_PMU_V3_FILTER provides the ability to let the VMM decide
> which PMU events are provided to the guest. Add a new option
> `kvm-pmu-filter` as -cpu sub-option to set the PMU Event Filtering.
> Without the filter, all PMU events are exposed from host to guest by
> default. The usage of the new sub-option can be found from the updated
> document (docs/system/arm/cpu-features.rst).
> 
> Here is an example which shows how to use the PMU Event Filtering, when
> we launch a guest by use kvm, add such command line:
> 
>   # qemu-system-aarch64 \
>         -accel kvm \
>         -cpu host,kvm-pmu-filter="D:0x11-0x11"
> 
> Since the first action is deny, we have a global allow policy. This
> filters out the cycle counter (event 0x11 being CPU_CYCLES).
> 
> And then in guest, use the perf to count the cycle:
> 
>   # perf stat sleep 1
> 
>    Performance counter stats for 'sleep 1':
> 
>               1.22 msec task-clock                       #    0.001 CPUs utilized
>                  1      context-switches                 #  820.695 /sec
>                  0      cpu-migrations                   #    0.000 /sec
>                 55      page-faults                      #   45.138 K/sec
>    <not supported>      cycles
>            1128954      instructions
>             227031      branches                         #  186.323 M/sec
>               8686      branch-misses                    #    3.83% of all branches
> 
>        1.002492480 seconds time elapsed
> 
>        0.001752000 seconds user
>        0.000000000 seconds sys
> 
> As we can see, the cycle counter has been disabled in the guest, but
> other pmu events do still work.
> 
> Reviewed-by: Sebastian Ott <sebott@xxxxxxxxxx>
> Signed-off-by: Shaoqin Huang <shahuang@xxxxxxxxxx>
Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>

Eric
> ---
> v6->v7:
>   - Check return value of sscanf.
>   - Improve the check condition.
> 
> v5->v6:
>   - Commit message improvement.
>   - Remove some unused code.
>   - Collect Reviewed-by, thanks Sebastian.
>   - Use g_auto(Gstrv) to replace the gchar **.          [Eric]
> 
> v4->v5:
>   - Change the kvm-pmu-filter as a -cpu sub-option.     [Eric]
>   - Comment tweak.                                      [Gavin]
>   - Rebase to the latest branch.
> 
> v3->v4:
>   - Fix the wrong check for pmu_filter_init.            [Sebastian]
>   - Fix multiple alignment issue.                       [Gavin]
>   - Report error by warn_report() instead of error_report(), and don't use
>   abort() since the PMU Event Filter is an add-on and best-effort feature.
>                                                         [Gavin]
>   - Add several missing {  } for single line of code.   [Gavin]
>   - Use the g_strsplit() to replace strtok().           [Gavin]
> 
> v2->v3:
>   - Improve commits message, use kernel doc wording, add more explaination on
>     filter example, fix some typo error.                [Eric]
>   - Add g_free() in kvm_arch_set_pmu_filter() to prevent memory leak. [Eric]
>   - Add more precise error message report.              [Eric]
>   - In options doc, add pmu-filter rely on KVM_ARM_VCPU_PMU_V3_FILTER support in
>     KVM.                                                [Eric]
> 
> v1->v2:
>   - Add more description for allow and deny meaning in 
>     commit message.                                     [Sebastian]
>   - Small improvement.                                  [Sebastian]
> 
>  docs/system/arm/cpu-features.rst | 23 +++++++++
>  target/arm/cpu.h                 |  3 ++
>  target/arm/kvm.c                 | 80 ++++++++++++++++++++++++++++++++
>  3 files changed, 106 insertions(+)
> 
> diff --git a/docs/system/arm/cpu-features.rst b/docs/system/arm/cpu-features.rst
> index a5fb929243..7c8f6a60ef 100644
> --- a/docs/system/arm/cpu-features.rst
> +++ b/docs/system/arm/cpu-features.rst
> @@ -204,6 +204,29 @@ the list of KVM VCPU features and their descriptions.
>    the guest scheduler behavior and/or be exposed to the guest
>    userspace.
>  
> +``kvm-pmu-filter``
> +  By default kvm-pmu-filter is disabled. This means that by default all pmu
> +  events will be exposed to guest.
> +
> +  KVM implements PMU Event Filtering to prevent a guest from being able to
> +  sample certain events. It depends on the KVM_ARM_VCPU_PMU_V3_FILTER
> +  attribute supported in KVM. It has the following format:
> +
> +  kvm-pmu-filter="{A,D}:start-end[;{A,D}:start-end...]"
> +
> +  The A means "allow" and D means "deny", start is the first event of the
> +  range and the end is the last one. The first registered range defines
> +  the global policy(global ALLOW if the first @action is DENY, global DENY
> +  if the first @action is ALLOW). The start and end only support hexadecimal
> +  format. For example:
> +
> +  kvm-pmu-filter="A:0x11-0x11;A:0x23-0x3a;D:0x30-0x30"
> +
> +  Since the first action is allow, we have a global deny policy. It
> +  will allow event 0x11 (The cycle counter), events 0x23 to 0x3a are
> +  also allowed except the event 0x30 which is denied, and all the other
> +  events are denied.
> +
>  TCG VCPU Features
>  =================
>  
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index 63f31e0d98..f7f2431755 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -948,6 +948,9 @@ struct ArchCPU {
>  
>      /* KVM steal time */
>      OnOffAuto kvm_steal_time;
> +
> +    /* KVM PMU Filter */
> +    char *kvm_pmu_filter;
>  #endif /* CONFIG_KVM */
>  
>      /* Uniprocessor system with MP extensions */
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index 81813030a5..5c62580d34 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -496,6 +496,22 @@ static void kvm_steal_time_set(Object *obj, bool value, Error **errp)
>      ARM_CPU(obj)->kvm_steal_time = value ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
>  }
>  
> +static char *kvm_pmu_filter_get(Object *obj, Error **errp)
> +{
> +    ARMCPU *cpu = ARM_CPU(obj);
> +
> +    return g_strdup(cpu->kvm_pmu_filter);
> +}
> +
> +static void kvm_pmu_filter_set(Object *obj, const char *pmu_filter,
> +                               Error **errp)
> +{
> +    ARMCPU *cpu = ARM_CPU(obj);
> +
> +    g_free(cpu->kvm_pmu_filter);
> +    cpu->kvm_pmu_filter = g_strdup(pmu_filter);
> +}
> +
>  /* KVM VCPU properties should be prefixed with "kvm-". */
>  void kvm_arm_add_vcpu_properties(ARMCPU *cpu)
>  {
> @@ -517,6 +533,12 @@ void kvm_arm_add_vcpu_properties(ARMCPU *cpu)
>                               kvm_steal_time_set);
>      object_property_set_description(obj, "kvm-steal-time",
>                                      "Set off to disable KVM steal time.");
> +
> +    object_property_add_str(obj, "kvm-pmu-filter", kvm_pmu_filter_get,
> +                            kvm_pmu_filter_set);
> +    object_property_set_description(obj, "kvm-pmu-filter",
> +                                    "PMU Event Filtering description for "
> +                                    "guest PMU. (default: NULL, disabled)");
>  }
>  
>  bool kvm_arm_pmu_supported(void)
> @@ -1706,6 +1728,62 @@ static bool kvm_arm_set_device_attr(ARMCPU *cpu, struct kvm_device_attr *attr,
>      return true;
>  }
>  
> +static void kvm_arm_pmu_filter_init(ARMCPU *cpu)
> +{
> +    static bool pmu_filter_init;
> +    struct kvm_pmu_event_filter filter;
> +    struct kvm_device_attr attr = {
> +        .group      = KVM_ARM_VCPU_PMU_V3_CTRL,
> +        .attr       = KVM_ARM_VCPU_PMU_V3_FILTER,
> +        .addr       = (uint64_t)&filter,
> +    };
> +    int i;
> +    g_auto(GStrv) event_filters;
> +
> +    if (!cpu->kvm_pmu_filter) {
> +        return;
> +    }
> +    if (kvm_vcpu_ioctl(CPU(cpu), KVM_HAS_DEVICE_ATTR, &attr)) {
> +        warn_report("The KVM doesn't support the PMU Event Filter!");
> +        return;
> +    }
> +
> +    /*
> +     * The filter only needs to be initialized through one vcpu ioctl and it
> +     * will affect all other vcpu in the vm.
> +     */
> +    if (pmu_filter_init) {
> +        return;
> +    } else {
> +        pmu_filter_init = true;
> +    }
> +
> +    event_filters = g_strsplit(cpu->kvm_pmu_filter, ";", -1);
> +    for (i = 0; event_filters[i]; i++) {
> +        unsigned short start = 0, end = 0;
> +        char act;
> +
> +        if (sscanf(event_filters[i], "%c:%hx-%hx", &act, &start, &end) != 3) {
> +            warn_report("Skipping invalid PMU filter %s", event_filters[i]);
> +            continue;
> +        }
> +
> +        if ((act != 'A' && act != 'D') || start > end) {
> +            warn_report("Skipping invalid PMU filter %s", event_filters[i]);
> +            continue;
> +        }
> +
> +        filter.base_event = start;
> +        filter.nevents = end - start + 1;
> +        filter.action = (act == 'A') ? KVM_PMU_EVENT_ALLOW :
> +                                       KVM_PMU_EVENT_DENY;
> +
> +        if (!kvm_arm_set_device_attr(cpu, &attr, "PMU_V3_FILTER")) {
> +            break;
> +        }
> +    }
> +}
> +
>  void kvm_arm_pmu_init(ARMCPU *cpu)
>  {
>      struct kvm_device_attr attr = {
> @@ -1716,6 +1794,8 @@ void kvm_arm_pmu_init(ARMCPU *cpu)
>      if (!cpu->has_pmu) {
>          return;
>      }
> +
> +    kvm_arm_pmu_filter_init(cpu);
>      if (!kvm_arm_set_device_attr(cpu, &attr, "PMU")) {
>          error_report("failed to init PMU");
>          abort();
> 
> base-commit: 760b4dcdddba4a40b9fa0eb78fdfc7eda7cb83d0





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