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. > @@ -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 > + * 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 > return; > > if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_32)
