Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

Sean Christopherson
2021-01-14T18:56:33Z

On Mon, Jan 04, 2021, Like Xu wrote:
> ---
> arch/x86/events/intel/ds.c | 62 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 62 insertions(+)
>
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index b47cc4226934..c499bdb58373 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -1721,6 +1721,65 @@ intel_pmu_save_and_restart_reload(struct perf_event *event, int count)
> return 0;
> }
>
> +/*
> + * We may be running with guest PEBS events created by KVM, and the
> + * PEBS records are logged into the guest's DS and invisible to host.
> + *
> + * In the case of guest PEBS overflow, we only trigger a fake event
> + * to emulate the PEBS overflow PMI for guest PBES counters in KVM.
> + * The guest will then vm-entry and check the guest DS area to read
> + * the guest PEBS records.
> + *
> + * The guest PEBS overflow PMI may be dropped when both the guest and
> + * the host use PEBS. Therefore, KVM will not enable guest PEBS once
> + * the host PEBS is enabled since it may bring a confused unknown NMI.
> + *
> + * The contents and other behavior of the guest event do not matter.
> + */
> +static int intel_pmu_handle_guest_pebs(struct cpu_hw_events *cpuc,
> + struct pt_regs *iregs,
> + struct debug_store *ds)
> +{
> + struct perf_sample_data data;
> + struct perf_event *event = NULL;
> + u64 guest_pebs_idxs = cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask;
> + int bit;
> +
> + /*
> + * Ideally, we should check guest DS to understand if it's
> + * a guest PEBS overflow PMI from guest PEBS counters.
> + * However, it brings high overhead to retrieve guest DS in host.
> + * So we check host DS instead for performance.
> + *
> + * If PEBS interrupt threshold on host is not exceeded in a NMI, there
> + * must be a PEBS overflow PMI generated from the guest PEBS counters.
> + * There is no ambiguity since the reported event in the PMI is guest
> + * only. It gets handled correctly on a case by case base for each event.
> + *
> + * Note: KVM disables the co-existence of guest PEBS and host PEBS.

By "KVM", do you mean KVM's loading of the MSRs provided by intel_guest_get_msrs()?
Because the PMU should really be the entity that controls guest vs. host. KVM
should just be a dumb pipe that handles the mechanics of how values are context
switch.

For example, commit 7099e2e1f4d9 ("KVM: VMX: disable PEBS before a guest entry"),
where KVM does an explicit WRMSR(PEBS_ENABLE) to (attempt to) force PEBS
quiescence, is flawed in that the PMU can re-enable PEBS after the WRMSR if a
PMI arrives between the WRMSR and VM-Enter (because VMX can't block NMIs). The
PMU really needs to be involved in the WRMSR workaround.

> + */
> + if (!guest_pebs_idxs || !in_nmi() ||

Are PEBS updates guaranteed to be isolated in both directions on relevant
hardware? By that I mean, will host updates be fully processed before VM-Enter
compeletes, and guest updates before VM-Exit completes? If that's the case,
then this path could be optimized to change the KVM invocation of the NMI
handler so that the "is this a guest PEBS PMI" check is done if and only if the
PMI originated from with the guest.

> + ds->pebs_index >= ds->pebs_interrupt_threshold)
> + return 0;
> +
> + for_each_set_bit(bit, (unsigned long *)&guest_pebs_idxs,
> + INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed) {
> +
> + event = cpuc->events[bit];
> + if (!event->attr.precise_ip)
> + continue;
> +
> + perf_sample_data_init(&data, 0, event->hw.last_period);
> + if (perf_event_overflow(event, &data, iregs))
> + x86_pmu_stop(event, 0);
> +
> + /* Inject one fake event is enough. */
> + return 1;
> + }
> +
> + return 0;
> +}