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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Feb 21, 2024 at 01:34:31AM -0500, 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"

snip

> @@ -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)");
>  }

Passing a string property, but....[1]

>  
>  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!");

If the user requested a filter and it can't be supported, QEMU
must exit with an error, not ignore the user's request.

> +        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;

Warning on user syntax errors is undesirable - it should be a fatal
error of the user gets this wrong.

> +        }
> +
> +        if ((act != 'A' && act != 'D') || start > end) {
> +            warn_report("Skipping invalid PMU filter %s", event_filters[i]);
> +            continue;

Likewise should be fatal.

> +        }
> +
> +        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;
> +        }
> +    }
> +}

..[1] then implementing a custom parser is rather a QEMU design anti-pattern,
especially when the proposed syntax is incapable of being mapped into the
normal QAPI syntax for a list of structs should we want to fully convert
-cpu to QAPI parsing later. I wonder if can we model this property with
QAPI now ?

With regards,
Daniel
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