Message-ID: <729c4b30-163c-4115-a380-14ece533a8b9@linux.intel.com>
Date: Tue, 23 Apr 2024 14:45:08 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: maobibo <maobibo@...ngson.cn>, Mingwei Zhang <mizhang@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
 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 4/23/2024 2:08 PM, maobibo wrote:
>
>
> On 2024/4/23 下午12:23, Mingwei Zhang wrote:
>> On Mon, Apr 22, 2024 at 8:55 PM maobibo <maobibo@...ngson.cn> wrote:
>>>
>>>
>>>
>>> On 2024/4/23 上午11:13, Mi, Dapeng wrote:
>>>>
>>>> On 4/23/2024 10:53 AM, maobibo wrote:
>>>>>
>>>>>
>>>>> 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@...gle.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.
>>>>
>>>> That's one issue this patchset tries to solve. Current new mediated 
>>>> x86
>>>> vPMU framework doesn't allow Host or Guest own the PMU HW resource
>>>> simultaneously. Only when there is no !exclude_guest event on host,
>>>> guest is allowed to exclusively own the PMU HW resource.
>>>>
>>>>
>>>>>
>>>>> 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.
>>>>
>>>> I didn't fully get your point. When IPI or timer interrupt reach, a
>>>> VM-exit is triggered to make CPU traps into host first and then the 
>>>> host
>>> yes, it is.
>>
>> This is correct. And this is one of the points that we had debated
>> internally whether we should do PMU context switch at vcpu loop
>> boundary or VM Enter/exit boundary. (host-level) timer interrupt can
>> force VM Exit, which I think happens every 4ms or 1ms, depending on
>> configuration.
>>
>> One of the key reasons we currently propose this is because it is the
>> same boundary as the legacy PMU, i.e., it would be simple to propose
>> from the perf subsystem perspective.
>>
>> Performance wise, doing PMU context switch at vcpu boundary would be
>> way better in general. But the downside is that perf sub-system lose
>> the capability to profile majority of the KVM code (functions) when
>> guest PMU is enabled.
>>
>>>
>>>> interrupt handler is called. Or are you complaining the executing
>>>> sequence of switching guest PMU MSRs and these interrupt handler?
>>> In our vPMU implementation, it is ok if vPMU is switched in vm exit
>>> path, however there is problem if vPMU is switched during vcpu thread
>>> sched-out/sched-in path since IPI/timer irq interrupt access pmu
>>> register in host mode.
>>
>> Oh, the IPI/timer irq handler will access PMU registers? I thought
>> only the host-level NMI handler will access the PMU MSRs since PMI is
>> registered under NMI.
>>
>> In that case, you should disable  IRQ during vcpu context switch. For
>> NMI, we prevent its handler from accessing the PMU registers. In
>> particular, we use a per-cpu variable to guard that. So, the
>> host-level PMI handler for perf sub-system will check the variable
>> before proceeding.
>
> perf core will access pmu hw in tick timer/hrtimer/ipi function call,
> such as function perf_event_task_tick() is called in tick timer, there
> are  event_function_call(event, __perf_event_xxx, &value) in file
> kernel/events/core.c.
>
> https://lore.kernel.org/lkml/20240417065236.500011-1-gaosong@loongson.cn/T/#m15aeb79fdc9ce72dd5b374edd6acdcf7a9dafcf4 
>

Just go through functions (not sure if all),  whether 
perf_event_task_tick() or the callbacks of event_function_call() would 
check the event->state first, if the event is in 
PERF_EVENT_STATE_INACTIVE, the PMU HW MSRs would not be touched really. 
In this new proposal, all host events with exclude_guest attribute would 
be put on PERF_EVENT_STATE_INACTIVE sate if guest own the PMU HW 
resource. So I think it's fine.


>
>
>>
>>>
>>> In general it will be better if the switch is done in vcpu thread
>>> sched-out/sched-in, else there is requirement to profile kvm
>>> hypervisor.Even there is such requirement, it is only one option. In
>>> most conditions, it will better if time of VM context exit is small.
>>>
>> Performance wise, agree, but there will be debate on perf
>> functionality loss at the host level.
>>
>> 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.
>>
>>>>
>>>>
>>>>>
>>>>> 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
>>>>>>>
>>>>>
>>>
>
>

Powered by blists - more mailing lists