Re: [RFC PATCH 00/41] KVM: x86/pmu: Introduce passthrough vPM

[Zhang, Xiong Y]

On 4/13/2024 2:32 AM, Sean Christopherson wrote:
> On Fri, Apr 12, 2024, Xiong Y Zhang wrote:
>>>> 2. NMI watchdog
>>>>    the perf event for NMI watchdog is a system wide cpu pinned event, it
>>>>    will be stopped also during vm running, but it doesn't have
>>>>    attr.exclude_guest=1, we add it in this RFC. But this still means NMI
>>>>    watchdog loses function during VM running.
>>>>
>>>>    Two candidates exist for replacing perf event of NMI watchdog:
>>>>    a. Buddy hardlock detector[3] may be not reliable to replace perf event.
>>>>    b. HPET-based hardlock detector [4] isn't in the upstream kernel.
>>>
>>> I think the simplest solution is to allow mediated PMU usage if and only if
>>> the NMI watchdog is disabled.  Then whether or not the host replaces the NMI
>>> watchdog with something else becomes an orthogonal discussion, i.e. not KVM's
>>> problem to solve.
>> Make sense. KVM should not affect host high priority work.
>> NMI watchdog is a client of perf and is a system wide perf event, perf can't
>> distinguish a system wide perf event is NMI watchdog or others, so how about
>> we extend this suggestion to all the system wide perf events ?  mediated PMU
>> is only allowed when all system wide perf events are disabled or non-exist at
>> vm creation.
> 
> What other kernel-driven system wide perf events are there?
does "kernel-driven" mean perf events created through perf_event_create_kernel_counter() like nmi_watchdog and kvm perf events ?
User can create system wide perf event through "perf record -e {} -a" also, I call it as user-driven system wide perf events.
Perf subsystem doesn't distinguish "kernel-driven" and "user-driven" system wide perf events.
> 
>> but NMI watchdog is usually enabled, this will limit mediated PMU usage.
> 
> I don't think it is at all unreasonable to require users that want optimal PMU
> virtualization to adjust their environment.  And we can and should document the
> tradeoffs and alternatives, e.g. so that users that want better PMU results don't
> need to re-discover all the "gotchas" on their own.
> 
> This would even be one of the rare times where I would be ok with a dmesg log.
> E.g. if KVM is loaded with enable_mediated_pmu=true, but there are system wide
> perf events, pr_warn() to explain the conflict and direct the user at documentation
> explaining how to make their system compatible with mediate PMU usage.> 
>>>> 3. Dedicated kvm_pmi_vector
>>>>    In emulated vPMU, host PMI handler notify KVM to inject a virtual
>>>>    PMI into guest when physical PMI belongs to guest counter. If the
>>>>    same mechanism is used in passthrough vPMU and PMI skid exists
>>>>    which cause physical PMI belonging to guest happens after VM-exit,
>>>>    then the host PMI handler couldn't identify this PMI belongs to
>>>>    host or guest.
>>>>    So this RFC uses a dedicated kvm_pmi_vector, PMI belonging to guest
>>>>    has this vector only. The PMI belonging to host still has an NMI
>>>>    vector.
>>>>
>>>>    Without considering PMI skid especially for AMD, the host NMI vector
>>>>    could be used for guest PMI also, this method is simpler and doesn't
>>>
>>> I don't see how multiplexing NMIs between guest and host is simpler.  At best,
>>> the complexity is a wash, just in different locations, and I highly doubt it's
>>> a wash.  AFAIK, there is no way to precisely know that an NMI came in via the
>>> LVTPC.
>> when kvm_intel.pt_mode=PT_MODE_HOST_GUEST, guest PT's PMI is a multiplexing
>> NMI between guest and host, we could extend guest PT's PMI framework to
>> mediated PMU. so I think this is simpler.
> 
> Heh, what do you mean by "this"?  Using a dedicated IRQ vector, or extending the
> PT framework of multiplexing NMI?
here "this" means "extending the PT framework of multiplexing NMI".

thanks
>