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Date: Fri, 26 Apr 2024 09:46:40 +0800
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Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU
To: Sean Christopherson <seanjc@google.com>,
 Kan Liang <kan.liang@linux.intel.com>
Cc: Mingwei Zhang <mizhang@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,
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 irogers@google.com, samantha.alt@intel.com, like.xu.linux@gmail.com,
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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@linux.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.

>
> 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.