Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

[Liang, Kan]

On 2024-04-25 11:12 p.m., Mingwei Zhang wrote:
>>>> 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. 

KVM doesn't need to save/restore the host state. Host perf has the
information and will reload the values whenever the host events are
rescheduled. But I think KVM should clear the registers used by the host
to prevent the value leaks to the guest.

> 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).
> 
> 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).

I'm not quit sure about the 4.
The new host-level perf must be an exclude_guest event. It should not be
scheduled when a guest is using the PMU. Why do we want to preempt the
guest PMU? The current implementation in perf doesn't schedule any
exclude_guest events when a guest is running.

Thanks,
Kan