Messages in this thread | | | Subject | Re: [PATCH v9 04/10] perf/x86: Keep LBR stack unchanged on the host for guest LBR event | From | "Xu, Like" <> | Date | Fri, 10 Apr 2020 11:10:38 +0800 |
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On 2020/4/10 0:45, Peter Zijlstra wrote: > On Fri, Mar 13, 2020 at 10:16:10AM +0800, Like Xu wrote: >> When a guest wants to use the LBR stack, its hypervisor creates a guest >> LBR event and let host perf schedules it. A new 'int guest_lbr_enabled' >> field in the "struct cpu_hw_events", is marked as true when perf adds >> a guest LBR event and false on deletion. >> >> The LBR stack msrs are accessible to the guest when its guest LBR event >> is scheduled in by the perf subsystem. Before scheduling out the event, >> we should avoid host changes on IA32_DEBUGCTLMSR or LBR_SELECT. Otherwise, >> some unexpected branch operations may interfere with guest behavior, >> pollute LBR records, and even cause host branch data leakage. In addition, >> the intel_pmu_lbr_read() on the host is also avoidable for guest usage. >> >> On v4 PMU or later, the LBR stack are frozen on the overflowed condition >> if Freeze_LBR_On_PMI is true and resume recording via acking LBRS_FROZEN >> to global status msr instead of re-enabling IA32_DEBUGCTL.LBR. So when a >> guest LBR event is running, the host PMI handler has to keep LBRS_FROZEN >> bit set (thus LBR being frozen) until the guest enables it. Otherwise, >> when the guest enters non-root mode, the LBR will start recording and >> the guest PMI handler code will also pollute the LBR stack. >> >> To ensure that guest LBR records are not lost during the context switch, >> the BRANCH_CALL_STACK flag should be configured in the 'branch_sample_type' >> for a guest LBR event because a callstack event could save/restore guest >> unread records with the help of intel_pmu_lbr_sched_task() naturally. >> >> However, the regular host LBR perf event doesn't save/restore LBR_SELECT, >> because it's configured in the LBR_enable() based on branch_sample_type. >> So when a guest LBR is running, the guest LBR_SELECT may changes for its >> own use and we have to add the LBR_SELECT save/restore to ensure what the >> guest LBR_SELECT value doesn't get lost during the context switching. > I had to read the patch before that made sense; I think it's mostly > there, but it can use a little help. Ah, thanks for your patient. This is good news for me that you did read the main part of the proposal changes in this version.
> > >> @@ -691,8 +714,12 @@ void intel_pmu_lbr_read(void) >> * >> * This could be smarter and actually check the event, >> * but this simple approach seems to work for now. >> + * >> + * And there is no need to read lbr here if a guest LBR event > There's 'lbr' and 'LBR' in the same sentence Yes, l'll fix it. > >> + * is using it, because the guest will read them on its own. >> */ >> - if (!cpuc->lbr_users || cpuc->lbr_users == cpuc->lbr_pebs_users) >> + if (!cpuc->lbr_users || cpuc->guest_lbr_enabled || >> + cpuc->lbr_users == cpuc->lbr_pebs_users) > indent fail Yes, l'll fix it. > >> return; >> >> if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_32)
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