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Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU
To: "Mi, Dapeng" <dapeng1.mi@linux.intel.com>,
 Sean Christopherson <seanjc@google.com>
Cc: Mingwei Zhang <mizhang@google.com>,
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From: maobibo <maobibo@loongson.cn>
Message-ID: <b5e97aa1-7683-4eff-e1e3-58ac98a8d719@loongson.cn>
Date: Tue, 23 Apr 2024 10:53:38 +0800
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On 2024/4/23 上午10:44, Mi, Dapeng wrote:
> 
> On 4/23/2024 9:01 AM, maobibo wrote:
>>
>>
>> On 2024/4/23 上午1:01, Sean Christopherson wrote:
>>> On Mon, Apr 22, 2024, maobibo wrote:
>>>> On 2024/4/16 上午6:45, Sean Christopherson wrote:
>>>>> On Mon, Apr 15, 2024, Mingwei Zhang wrote:
>>>>>> On Mon, Apr 15, 2024 at 10:38 AM Sean Christopherson 
>>>>>> <seanjc@google.com> wrote:
>>>>>>> One my biggest complaints with the current vPMU code is that the 
>>>>>>> roles and
>>>>>>> responsibilities between KVM and perf are poorly defined, which 
>>>>>>> leads to suboptimal
>>>>>>> and hard to maintain code.
>>>>>>>
>>>>>>> Case in point, I'm pretty sure leaving guest values in PMCs 
>>>>>>> _would_ leak guest
>>>>>>> state to userspace processes that have RDPMC permissions, as the 
>>>>>>> PMCs might not
>>>>>>> be dirty from perf's perspective (see perf_clear_dirty_counters()).
>>>>>>>
>>>>>>> Blindly clearing PMCs in KVM "solves" that problem, but in doing 
>>>>>>> so makes the
>>>>>>> overall code brittle because it's not clear whether KVM _needs_ 
>>>>>>> to clear PMCs,
>>>>>>> or if KVM is just being paranoid.
>>>>>>
>>>>>> So once this rolls out, perf and vPMU are clients directly to PMU HW.
>>>>>
>>>>> I don't think this is a statement we want to make, as it opens a 
>>>>> discussion
>>>>> that we won't win.  Nor do I think it's one we *need* to make.  KVM 
>>>>> doesn't need
>>>>> to be on equal footing with perf in terms of owning/managing PMU 
>>>>> hardware, KVM
>>>>> just needs a few APIs to allow faithfully and accurately 
>>>>> virtualizing a guest PMU.
>>>>>
>>>>>> Faithful cleaning (blind cleaning) has to be the baseline
>>>>>> implementation, until both clients agree to a "deal" between them.
>>>>>> Currently, there is no such deal, but I believe we could have one via
>>>>>> future discussion.
>>>>>
>>>>> What I am saying is that there needs to be a "deal" in place before 
>>>>> this code
>>>>> is merged.  It doesn't need to be anything fancy, e.g. perf can 
>>>>> still pave over
>>>>> PMCs it doesn't immediately load, as opposed to using 
>>>>> cpu_hw_events.dirty to lazily
>>>>> do the clearing.  But perf and KVM need to work together from the 
>>>>> get go, ie. I
>>>>> don't want KVM doing something without regard to what perf does, 
>>>>> and vice versa.
>>>>>
>>>> There is similar issue on LoongArch vPMU where vm can directly pmu 
>>>> hardware
>>>> and pmu hw is shard with guest and host. Besides context switch 
>>>> there are
>>>> other places where perf core will access pmu hw, such as tick
>>>> timer/hrtimer/ipi function call, and KVM can only intercept context 
>>>> switch.
>>>
>>> Two questions:
>>>
>>>   1) Can KVM prevent the guest from accessing the PMU?
>>>
>>>   2) If so, KVM can grant partial access to the PMU, or is it all or 
>>> nothing?
>>>
>>> If the answer to both questions is "yes", then it sounds like 
>>> LoongArch *requires*
>>> mediated/passthrough support in order to virtualize its PMU.
>>
>> Hi Sean,
>>
>> Thank for your quick response.
>>
>> yes, kvm can prevent guest from accessing the PMU and grant partial or 
>> all to access to the PMU. Only that if one pmu event is granted to VM, 
>> host can not access this pmu event again. There must be pmu event 
>> switch if host want to.
> 
> PMU event is a software entity which won't be shared. did you mean if a 
> PMU HW counter is granted to VM, then Host can't access the PMU HW 
> counter, right?
yes, if PMU HW counter/control is granted to VM. The value comes from 
guest, and is not meaningful for host.  Host pmu core does not know that 
it is granted to VM, host still think that it owns pmu.

Just like FPU register, it is shared by VM and host during different 
time and it is lately switched. But if IPI or timer interrupt uses FPU 
register on host, there will be the same issue.

Regards
Bibo Mao
> 
> 
>>
>>>
>>>> Can we add callback handler in structure kvm_guest_cbs?  just like 
>>>> this:
>>>> @@ -6403,6 +6403,7 @@ static struct perf_guest_info_callbacks 
>>>> kvm_guest_cbs
>>>> = {
>>>>          .state                  = kvm_guest_state,
>>>>          .get_ip                 = kvm_guest_get_ip,
>>>>          .handle_intel_pt_intr   = NULL,
>>>> +       .lose_pmu               = kvm_guest_lose_pmu,
>>>>   };
>>>>
>>>> By the way, I do not know should the callback handler be triggered 
>>>> in perf
>>>> core or detailed pmu hw driver. From ARM pmu hw driver, it is 
>>>> triggered in
>>>> pmu hw driver such as function kvm_vcpu_pmu_resync_el0,
>>>> but I think it will be better if it is done in perf core.
>>>
>>> I don't think we want to take the approach of perf and KVM guests 
>>> "fighting" over
>>> the PMU.  That's effectively what we have today, and it's a mess for 
>>> KVM because
>>> it's impossible to provide consistent, deterministic behavior for the 
>>> guest.  And
>>> it's just as messy for perf, which ends up having wierd, cumbersome 
>>> flows that
>>> exists purely to try to play nice with KVM.
>> With existing pmu core code, in tick timer interrupt or IPI function 
>> call interrupt pmu hw may be accessed by host when VM is running and 
>> pmu is already granted to guest. KVM can not intercept host IPI/timer 
>> interrupt, there is no pmu context switch, there will be problem.
>>
>> Regards
>> Bibo Mao
>>