From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Mingwei Zhang <mizhang@google.com>
Cc: "Mi, Dapeng" <dapeng1.mi@linux.intel.com>,
Sean Christopherson <seanjc@google.com>,
maobibo <maobibo@loongson.cn>,
Xiong Zhang <xiong.y.zhang@linux.intel.com>,
pbonzini@redhat.com, peterz@infradead.org, kan.liang@intel.com,
zhenyuw@linux.intel.com, jmattson@google.com,
kvm@vger.kernel.org, linux-perf-users@vger.kernel.org,
linux-kernel@vger.kernel.org, zhiyuan.lv@intel.com,
eranian@google.com, irogers@google.com, samantha.alt@intel.com,
like.xu.linux@gmail.com, chao.gao@intel.com
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU
Date: Thu, 25 Apr 2024 16:43:39 -0400 [thread overview]
Message-ID: <42acf1fc-1603-4ac5-8a09-edae2d85963d@linux.intel.com> (raw)
In-Reply-To: <CAL715W+zeqKenPLP2Fm9u_BkGRKAk-mncsOxrg=EKs74qK5f1Q@mail.gmail.com>
On 2024-04-25 4:16 p.m., Mingwei Zhang wrote:
> On Thu, Apr 25, 2024 at 9:13 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>
>>
>>
>> On 2024-04-25 12:24 a.m., Mingwei Zhang wrote:
>>> On Wed, Apr 24, 2024 at 8:56 PM Mi, Dapeng <dapeng1.mi@linux.intel.com> wrote:
>>>>
>>>>
>>>> On 4/24/2024 11:00 PM, Sean Christopherson wrote:
>>>>> On Wed, Apr 24, 2024, Dapeng Mi wrote:
>>>>>> On 4/24/2024 1:02 AM, Mingwei Zhang wrote:
>>>>>>>>> Maybe, (just maybe), it is possible to do PMU context switch at vcpu
>>>>>>>>> boundary normally, but doing it at VM Enter/Exit boundary when host is
>>>>>>>>> profiling KVM kernel module. So, dynamically adjusting PMU context
>>>>>>>>> switch location could be an option.
>>>>>>>> If there are two VMs with pmu enabled both, however host PMU is not
>>>>>>>> enabled. PMU context switch should be done in vcpu thread sched-out path.
>>>>>>>>
>>>>>>>> If host pmu is used also, we can choose whether PMU switch should be
>>>>>>>> done in vm exit path or vcpu thread sched-out path.
>>>>>>>>
>>>>>>> host PMU is always enabled, ie., Linux currently does not support KVM
>>>>>>> PMU running standalone. I guess what you mean is there are no active
>>>>>>> perf_events on the host side. Allowing a PMU context switch drifting
>>>>>>> from vm-enter/exit boundary to vcpu loop boundary by checking host
>>>>>>> side events might be a good option. We can keep the discussion, but I
>>>>>>> won't propose that in v2.
>>>>>> I suspect if it's really doable to do this deferring. This still makes host
>>>>>> lose the most of capability to profile KVM. Per my understanding, most of
>>>>>> KVM overhead happens in the vcpu loop, exactly speaking in VM-exit handling.
>>>>>> We have no idea when host want to create perf event to profile KVM, it could
>>>>>> be at any time.
>>>>> No, the idea is that KVM will load host PMU state asap, but only when host PMU
>>>>> state actually needs to be loaded, i.e. only when there are relevant host events.
>>>>>
>>>>> If there are no host perf events, KVM keeps guest PMU state loaded for the entire
>>>>> KVM_RUN loop, i.e. provides optimal behavior for the guest. But if a host perf
>>>>> events exists (or comes along), the KVM context switches PMU at VM-Enter/VM-Exit,
>>>>> i.e. lets the host profile almost all of KVM, at the cost of a degraded experience
>>>>> for the guest while host perf events are active.
>>>>
>>>> I see. So KVM needs to provide a callback which needs to be called in
>>>> the IPI handler. The KVM callback needs to be called to switch PMU state
>>>> before perf really enabling host event and touching PMU MSRs. And only
>>>> the perf event with exclude_guest attribute is allowed to create on
>>>> host. Thanks.
>>>
>>> Do we really need a KVM callback? I think that is one option.
>>>
>>> Immediately after VMEXIT, KVM will check whether there are "host perf
>>> events". If so, do the PMU context switch immediately. Otherwise, keep
>>> deferring the context switch to the end of vPMU loop.
>>>
>>> Detecting if there are "host perf events" would be interesting. The
>>> "host perf events" refer to the perf_events on the host that are
>>> active and assigned with HW counters and that are saved when context
>>> switching to the guest PMU. I think getting those events could be done
>>> by fetching the bitmaps in cpuc.
>>
>> The cpuc is ARCH specific structure. I don't think it can be get in the
>> generic code. You probably have to implement ARCH specific functions to
>> fetch the bitmaps. It probably won't worth it.
>>
>> You may check the pinned_groups and flexible_groups to understand if
>> there are host perf events which may be scheduled when VM-exit. But it
>> will not tell the idx of the counters which can only be got when the
>> host event is really scheduled.
>>
>>> I have to look into the details. But
>>> at the time of VMEXIT, kvm should already have that information, so it
>>> can immediately decide whether to do the PMU context switch or not.
>>>
>>> oh, but when the control is executing within the run loop, a
>>> host-level profiling starts, say 'perf record -a ...', it will
>>> generate an IPI to all CPUs. Maybe that's when we need a callback so
>>> the KVM guest PMU context gets preempted for the host-level profiling.
>>> Gah..
>>>
>>> hmm, not a fan of that. That means the host can poke the guest PMU
>>> context at any time and cause higher overhead. But I admit it is much
>>> better than the current approach.
>>>
>>> The only thing is that: any command like 'perf record/stat -a' shot in
>>> dark corners of the host can preempt guest PMUs of _all_ running VMs.
>>> So, to alleviate that, maybe a module parameter that disables this
>>> "preemption" is possible? This should fit scenarios where we don't
>>> want guest PMU to be preempted outside of the vCPU loop?
>>>
>>
>> It should not happen. For the current implementation, perf rejects all
>> the !exclude_guest system-wide event creation if a guest with the vPMU
>> is running.
>> However, it's possible to create an exclude_guest system-wide event at
>> any time. KVM cannot use the information from the VM-entry to decide if
>> there will be active perf events in the VM-exit.
>
> Hmm, why not? If there is any exclude_guest system-wide event,
> perf_guest_enter() can return something to tell KVM "hey, some active
> host events are swapped out. they are originally in counter #2 and
> #3". If so, at the time when perf_guest_enter() returns, KVM will ack
> that and keep it in its pmu data structure.
I think it's possible that someone creates !exclude_guest event after
the perf_guest_enter(). The stale information is saved in the KVM. Perf
will schedule the event in the next perf_guest_exit(). KVM will not know it.
>
> Now, when doing context switching back to host at just VMEXIT, KVM
> will check this data and see if host perf context has something active
> (of course, they are all exclude_guest events). If not, deferring the
> context switch to vcpu boundary. Otherwise, do the proper PMU context
> switching by respecting the occupied counter positions on the host
> side, i.e., avoid doubling the work on the KVM side.
>
> Kan, any suggestion on the above approach?
I think we can only know the accurate event list at perf_guest_exit().
You may check the pinned_groups and flexible_groups, which tell if there
are candidate events.
> Totally understand that
> there might be some difficulty, since perf subsystem works in several
> layers and obviously fetching low-level mapping is arch specific work.
> If that is difficult, we can split the work in two phases: 1) phase
> #1, just ask perf to tell kvm if there are active exclude_guest events
> swapped out; 2) phase #2, ask perf to tell their (low-level) counter
> indices.
>
If you want an accurate counter mask, the changes in the arch specific
code is required. Two phases sound good to me.
Besides perf changes, I think the KVM should also track which counters
need to be saved/restored. The information can be get from the EventSel
interception.
Thanks,
Kan
>>
>> The perf_guest_exit() will reload the host state. It's impossible to
>> save the guest state after that. We may need a KVM callback. So perf can
>> tell KVM whether to save the guest state before perf reloads the host state.
>>
>> Thanks,
>> Kan
>>>>
>>>>
>>>>>
>>>>> My original sketch: https://lore.kernel.org/all/ZR3eNtP5IVAHeFNC@googlecom
>>>
>
next prev parent reply other threads:[~2024-04-25 20:43 UTC|newest]
Thread overview: 181+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-26 8:54 [RFC PATCH 00/41] KVM: x86/pmu: Introduce passthrough vPM Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 01/41] perf: x86/intel: Support PERF_PMU_CAP_VPMU_PASSTHROUGH Xiong Zhang
2024-04-11 17:04 ` Sean Christopherson
2024-04-11 17:21 ` Liang, Kan
2024-04-11 17:24 ` Jim Mattson
2024-04-11 17:46 ` Sean Christopherson
2024-04-11 19:13 ` Liang, Kan
2024-04-11 20:43 ` Sean Christopherson
2024-04-11 21:04 ` Liang, Kan
2024-04-11 19:32 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 02/41] perf: Support guest enter/exit interfaces Xiong Zhang
2024-03-20 16:40 ` Raghavendra Rao Ananta
2024-03-20 17:12 ` Liang, Kan
2024-04-11 18:06 ` Sean Christopherson
2024-04-11 19:53 ` Liang, Kan
2024-04-12 19:17 ` Sean Christopherson
2024-04-12 20:56 ` Liang, Kan
2024-04-15 16:03 ` Liang, Kan
2024-04-16 5:34 ` Zhang, Xiong Y
2024-04-16 12:48 ` Liang, Kan
2024-04-17 9:42 ` Zhang, Xiong Y
2024-04-18 16:11 ` Sean Christopherson
2024-04-19 1:37 ` Zhang, Xiong Y
2024-04-26 4:09 ` Zhang, Xiong Y
2024-01-26 8:54 ` [RFC PATCH 03/41] perf: Set exclude_guest onto nmi_watchdog Xiong Zhang
2024-04-11 18:56 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 04/41] perf: core/x86: Add support to register a new vector for PMI handling Xiong Zhang
2024-04-11 17:10 ` Sean Christopherson
2024-04-11 19:05 ` Sean Christopherson
2024-04-12 3:56 ` Zhang, Xiong Y
2024-04-13 1:17 ` Mi, Dapeng
2024-01-26 8:54 ` [RFC PATCH 05/41] KVM: x86/pmu: Register PMI handler for passthrough PMU Xiong Zhang
2024-04-11 19:07 ` Sean Christopherson
2024-04-12 5:44 ` Zhang, Xiong Y
2024-01-26 8:54 ` [RFC PATCH 06/41] perf: x86: Add function to switch PMI handler Xiong Zhang
2024-04-11 19:17 ` Sean Christopherson
2024-04-11 19:34 ` Sean Christopherson
2024-04-12 6:03 ` Zhang, Xiong Y
2024-04-12 5:57 ` Zhang, Xiong Y
2024-01-26 8:54 ` [RFC PATCH 07/41] perf/x86: Add interface to reflect virtual LVTPC_MASK bit onto HW Xiong Zhang
2024-04-11 19:21 ` Sean Christopherson
2024-04-12 6:17 ` Zhang, Xiong Y
2024-01-26 8:54 ` [RFC PATCH 08/41] KVM: x86/pmu: Add get virtual LVTPC_MASK bit function Xiong Zhang
2024-04-11 19:22 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 09/41] perf: core/x86: Forbid PMI handler when guest own PMU Xiong Zhang
2024-04-11 19:26 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 10/41] perf: core/x86: Plumb passthrough PMU capability from x86_pmu to x86_pmu_cap Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 11/41] KVM: x86/pmu: Introduce enable_passthrough_pmu module parameter and propage to KVM instance Xiong Zhang
2024-04-11 20:54 ` Sean Christopherson
2024-04-11 21:03 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 12/41] KVM: x86/pmu: Plumb through passthrough PMU to vcpu for Intel CPUs Xiong Zhang
2024-04-11 20:57 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 13/41] KVM: x86/pmu: Add a helper to check if passthrough PMU is enabled Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 14/41] KVM: x86/pmu: Allow RDPMC pass through Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 15/41] KVM: x86/pmu: Manage MSR interception for IA32_PERF_GLOBAL_CTRL Xiong Zhang
2024-04-11 21:21 ` Sean Christopherson
2024-04-11 22:30 ` Jim Mattson
2024-04-11 23:27 ` Sean Christopherson
2024-04-13 2:10 ` Mi, Dapeng
2024-01-26 8:54 ` [RFC PATCH 16/41] KVM: x86/pmu: Create a function prototype to disable MSR interception Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 17/41] KVM: x86/pmu: Implement pmu function for Intel CPU " Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 18/41] KVM: x86/pmu: Intercept full-width GP counter MSRs by checking with perf capabilities Xiong Zhang
2024-04-11 21:23 ` Sean Christopherson
2024-04-11 21:50 ` Jim Mattson
2024-04-12 16:01 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 19/41] KVM: x86/pmu: Whitelist PMU MSRs for passthrough PMU Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 20/41] KVM: x86/pmu: Introduce PMU operation prototypes for save/restore PMU context Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 21/41] KVM: x86/pmu: Introduce function prototype for Intel CPU to " Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 22/41] x86: Introduce MSR_CORE_PERF_GLOBAL_STATUS_SET for passthrough PMU Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU Xiong Zhang
2024-04-11 21:26 ` Sean Christopherson
2024-04-13 2:29 ` Mi, Dapeng
2024-04-11 21:44 ` Sean Christopherson
2024-04-11 22:19 ` Jim Mattson
2024-04-11 23:31 ` Sean Christopherson
2024-04-13 3:19 ` Mi, Dapeng
2024-04-13 3:03 ` Mi, Dapeng
2024-04-13 3:34 ` Mingwei Zhang
2024-04-13 4:25 ` Mi, Dapeng
2024-04-15 6:06 ` Mingwei Zhang
2024-04-15 10:04 ` Mi, Dapeng
2024-04-15 16:44 ` Mingwei Zhang
2024-04-15 17:38 ` Sean Christopherson
2024-04-15 17:54 ` Mingwei Zhang
2024-04-15 22:45 ` Sean Christopherson
2024-04-22 2:14 ` maobibo
2024-04-22 17:01 ` Sean Christopherson
2024-04-23 1:01 ` maobibo
2024-04-23 2:44 ` Mi, Dapeng
2024-04-23 2:53 ` maobibo
2024-04-23 3:13 ` Mi, Dapeng
2024-04-23 3:26 ` maobibo
2024-04-23 3:59 ` Mi, Dapeng
2024-04-23 3:55 ` maobibo
2024-04-23 4:23 ` Mingwei Zhang
2024-04-23 6:08 ` maobibo
2024-04-23 6:45 ` Mi, Dapeng
2024-04-23 7:10 ` Mingwei Zhang
2024-04-23 8:24 ` Mi, Dapeng
2024-04-23 8:51 ` maobibo
2024-04-23 16:50 ` Mingwei Zhang
2024-04-23 12:12 ` maobibo
2024-04-23 17:02 ` Mingwei Zhang
2024-04-24 1:07 ` maobibo
2024-04-24 8:18 ` Mi, Dapeng
2024-04-24 15:00 ` Sean Christopherson
2024-04-25 3:55 ` Mi, Dapeng
2024-04-25 4:24 ` Mingwei Zhang
2024-04-25 16:13 ` Liang, Kan
2024-04-25 20:16 ` Mingwei Zhang
2024-04-25 20:43 ` Liang, Kan [this message]
2024-04-25 21:46 ` Sean Christopherson
2024-04-26 1:46 ` Mi, Dapeng
2024-04-26 3:12 ` Mingwei Zhang
2024-04-26 4:02 ` Mi, Dapeng
2024-04-26 4:46 ` Mingwei Zhang
2024-04-26 14:09 ` Liang, Kan
2024-04-26 18:41 ` Mingwei Zhang
2024-04-26 19:06 ` Liang, Kan
2024-04-26 19:46 ` Sean Christopherson
2024-04-27 3:04 ` Mingwei Zhang
2024-04-28 0:58 ` Mi, Dapeng
2024-04-28 6:01 ` Mingwei Zhang
2024-04-29 17:44 ` Sean Christopherson
2024-05-01 17:43 ` Mingwei Zhang
2024-05-01 18:00 ` Liang, Kan
2024-05-01 20:36 ` Sean Christopherson
2024-04-29 13:08 ` Liang, Kan
2024-04-26 13:53 ` Liang, Kan
2024-04-26 1:50 ` Mi, Dapeng
2024-04-18 21:21 ` Mingwei Zhang
2024-04-18 21:41 ` Mingwei Zhang
2024-04-19 1:02 ` Mi, Dapeng
2024-01-26 8:54 ` [RFC PATCH 24/41] KVM: x86/pmu: Zero out unexposed Counters/Selectors to avoid information leakage Xiong Zhang
2024-04-11 21:36 ` Sean Christopherson
2024-04-11 21:56 ` Jim Mattson
2024-01-26 8:54 ` [RFC PATCH 25/41] KVM: x86/pmu: Introduce macro PMU_CAP_PERF_METRICS Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 26/41] KVM: x86/pmu: Add host_perf_cap field in kvm_caps to record host PMU capability Xiong Zhang
2024-04-11 21:49 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 27/41] KVM: x86/pmu: Clear PERF_METRICS MSR for guest Xiong Zhang
2024-04-11 21:50 ` Sean Christopherson
2024-04-13 3:29 ` Mi, Dapeng
2024-01-26 8:54 ` [RFC PATCH 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL_CTRL at VM boundary Xiong Zhang
2024-04-11 21:54 ` Sean Christopherson
2024-04-11 22:10 ` Jim Mattson
2024-04-11 22:54 ` Sean Christopherson
2024-04-11 23:08 ` Jim Mattson
2024-01-26 8:54 ` [RFC PATCH 29/41] KVM: x86/pmu: Exclude existing vLBR logic from the passthrough PMU Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 30/41] KVM: x86/pmu: Switch PMI handler at KVM context switch boundary Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 31/41] KVM: x86/pmu: Call perf_guest_enter() at PMU context switch Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 32/41] KVM: x86/pmu: Add support for PMU context switch at VM-exit/enter Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 33/41] KVM: x86/pmu: Make check_pmu_event_filter() an exported function Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 34/41] KVM: x86/pmu: Intercept EVENT_SELECT MSR Xiong Zhang
2024-04-11 21:55 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 35/41] KVM: x86/pmu: Allow writing to event selector for GP counters if event is allowed Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 36/41] KVM: x86/pmu: Intercept FIXED_CTR_CTRL MSR Xiong Zhang
2024-04-11 21:56 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 37/41] KVM: x86/pmu: Allow writing to fixed counter selector if counter is exposed Xiong Zhang
2024-04-11 22:03 ` Sean Christopherson
2024-04-13 4:12 ` Mi, Dapeng
2024-01-26 8:54 ` [RFC PATCH 38/41] KVM: x86/pmu: Introduce PMU helper to increment counter Xiong Zhang
2024-01-26 8:54 ` [RFC PATCH 39/41] KVM: x86/pmu: Implement emulated counter increment for passthrough PMU Xiong Zhang
2024-04-11 23:12 ` Sean Christopherson
2024-04-11 23:17 ` Sean Christopherson
2024-01-26 8:54 ` [RFC PATCH 40/41] KVM: x86/pmu: Separate passthrough PMU logic in set/get_msr() from non-passthrough vPMU Xiong Zhang
2024-04-11 23:18 ` Sean Christopherson
2024-04-18 21:54 ` Mingwei Zhang
2024-01-26 8:54 ` [RFC PATCH 41/41] KVM: nVMX: Add nested virtualization support for passthrough PMU Xiong Zhang
2024-04-11 23:21 ` Sean Christopherson
2024-04-11 17:03 ` [RFC PATCH 00/41] KVM: x86/pmu: Introduce passthrough vPM Sean Christopherson
2024-04-12 2:19 ` Zhang, Xiong Y
2024-04-12 18:32 ` Sean Christopherson
2024-04-15 1:06 ` Zhang, Xiong Y
2024-04-15 15:05 ` Sean Christopherson
2024-04-16 5:11 ` Zhang, Xiong Y
2024-04-18 20:46 ` Mingwei Zhang
2024-04-18 21:52 ` Mingwei Zhang
2024-04-19 19:14 ` Sean Christopherson
2024-04-19 22:02 ` Mingwei Zhang
2024-04-11 23:25 ` Sean Christopherson
2024-04-11 23:56 ` Mingwei Zhang
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