Date: Fri, 26 Apr 2024 12:02:55 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
 Kan Liang <kan.liang@...ux.intel.com>, maobibo <maobibo@...ngson.cn>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>, pbonzini@...hat.com,
 peterz@...radead.org, kan.liang@...el.com, zhenyuw@...ux.intel.com,
 jmattson@...gle.com, kvm@...r.kernel.org, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, zhiyuan.lv@...el.com, eranian@...gle.com,
 irogers@...gle.com, samantha.alt@...el.com, like.xu.linux@...il.com,
 chao.gao@...el.com
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU


On 4/26/2024 11:12 AM, Mingwei Zhang wrote:
> On Thu, Apr 25, 2024 at 6:46 PM Mi, Dapeng <dapeng1.mi@...ux.intel.com> wrote:
>>
>> On 4/26/2024 5:46 AM, Sean Christopherson wrote:
>>> On Thu, Apr 25, 2024, Kan Liang wrote:
>>>> On 2024-04-25 4:16 p.m., Mingwei Zhang wrote:
>>>>> On Thu, Apr 25, 2024 at 9:13 AM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>>>>>> 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
>>> I assume you mean an exclude_guest=1 event?  Because perf should be in a state
>>> where it rejects exclude_guest=0 events.
>> Suppose should be exclude_guest=1 event, the perf event without
>> exclude_guest attribute would be blocked to create in the v2 patches
>> which we are working on.
>>
>>
>>>> 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.
>>> Ya, the creation of an event on a CPU that currently has guest PMU state loaded
>>> is what I had in mind when I suggested a callback in my sketch:
>>>
>>>    :  D. Add a perf callback that is invoked from IRQ context when perf wants to
>>>    :     configure a new PMU-based events, *before* actually programming the MSRs,
>>>    :     and have KVM's callback put the guest PMU state
>>
>> when host creates a perf event with exclude_guest attribute which is
>> used to profile KVM/VMM user space, the vCPU process could work at three
>> places.
>>
>> 1. in guest state (non-root mode)
>>
>> 2. inside vcpu-loop
>>
>> 3. outside vcpu-loop
>>
>> Since the PMU state has already been switched to host state, we don't
>> need to consider the case 3 and only care about cases 1 and 2.
>>
>> when host creates a perf event with exclude_guest attribute to profile
>> KVM/VMM user space,  an IPI is triggered to enable the perf event
>> eventually like the following code shows.
>>
>> event_function_call(event, __perf_event_enable, NULL);
>>
>> For case 1,  a vm-exit is triggered and KVM starts to process the
>> vm-exit and then run IPI irq handler, exactly speaking
>> __perf_event_enable() to enable the perf event.
>>
>> For case 2, the IPI irq handler would preempt the vcpu-loop and call
>> __perf_event_enable() to enable the perf event.
>>
>> So IMO KVM just needs to provide a callback to switch guest/host PMU
>> state, and __perf_event_enable() calls this callback before really
>> touching PMU MSRs.
> ok, in this case, do we still need KVM to query perf if there are
> active exclude_guest events? yes? Because there is an ordering issue.
> The above suggests that the host-level perf profiling comes when a VM
> is already running, there is an IPI that can invoke the callback and
> trigger preemption. In this case, KVM should switch the context from
> guest to host. What if it is the other way around, ie., host-level
> profiling runs first and then VM runs?
>
> In this case, just before entering the vcpu loop, kvm should check
> whether there is an active host event and save that into a pmu data
> structure. If none, do the context switch early (so that KVM saves a
> huge amount of unnecessary PMU context switches in the future).
> Otherwise, keep the host PMU context until vm-enter. At the time of
> vm-exit, do the check again using the data stored in pmu structure. If
> there is an active event do the context switch to the host PMU,
> otherwise defer that until exiting the vcpu loop. Of course, in the
> meantime, if there is any perf profiling started causing the IPI, the
> irq handler calls the callback, preempting the guest PMU context. If
> that happens, at the time of exiting the vcpu boundary, PMU context
> switch is skipped since it is already done. Of course, note that the
> irq could come at any time, so the PMU context switch in all 4
> locations need to check the state flag (and skip the context switch if
> needed).
>
> So this requires vcpu->pmu has two pieces of state information: 1) the
> flag similar to TIF_NEED_FPU_LOAD; 2) host perf context info (phase #1
> just a boolean; phase #2, bitmap of occupied counters).

I still had no chance to look at the details about vFPU implementation, 
currently I have no idea what we need exactly on vPMU side, a flag or a 
callback. Anyway, that's just implementation details, we can look at it 
when starting to implement it.

>
> This is a non-trivial optimization on the PMU context switch. I am
> thinking about splitting them into the following phases:
>
> 1) lazy PMU context switch, i.e., wait until the guest touches PMU MSR
> for the 1st time.
> 2) fast PMU context switch on KVM side, i.e., KVM checking event
> selector value (enable/disable) and selectively switch PMU state
> (reducing rd/wr msrs)
> 3) dynamic PMU context boundary, ie., KVM can dynamically choose PMU
> context switch boundary depending on existing active host-level
> events.
> 3.1) more accurate dynamic PMU context switch, ie., KVM checking
> host-level counter position and further reduces the number of msr
> accesses.
> 4) guest PMU context preemption, i.e., any new host-level perf
> profiling can immediately preempt the guest PMU in the vcpu loop
> (instead of waiting for the next PMU context switch in KVM).

Great! we have a whole clear picture about the optimization right now. 
BTW, the optimization 1 and 2 are already on our original to-do list. We 
plan to do it after RFC v2 is ready.


>
> Thanks.
> -Mingwei
>>> It's a similar idea to TIF_NEED_FPU_LOAD, just that instead of a common chunk of
>>> kernel code swapping out the guest state (kernel_fpu_begin()), it's a callback
>>> into KVM.

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