Re: [PATCH v9 04/10] perf/x86: Keep LBR stack unchanged on the host for guest LBR event

[Xu, Like]

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)