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

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Message-ID: <20200409164545.GE20713@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Apr 2020 18:45:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Liran Alon <liran.alon@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Liang Kan <kan.liang@...ux.intel.com>,
        Wei Wang <wei.w.wang@...el.com>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v9 04/10] perf/x86: Keep LBR stack unchanged on the host
 for guest LBR event

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)