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

[Mingwei Zhang]

On Thu, Apr 18, 2024, Mingwei Zhang wrote:
> On Thu, Apr 11, 2024, Sean Christopherson wrote:
> > <bikeshed>
> > 
> > I think we should call this a mediated PMU, not a passthrough PMU.  KVM still
> > emulates the control plane (controls and event selectors), while the data is
> > fully passed through (counters).
> > 
> > </bikeshed>
> Sean,
> 
> I feel "mediated PMU" seems to be a little bit off the ..., no? In
> KVM, almost all of features are mediated. In our specific case, the
> legacy PMU is mediated by KVM and perf subsystem on the host. In new
> design, it is mediated by KVM only.
> 
> We intercept the control plan in current design, but the only thing
> we do is the event filtering. No fancy code change to emulate the control
> registers. So, it is still a passthrough logic.
> 
> In some (rare) business cases, I think maybe we could fully passthrough
> the control plan as well. For instance, sole-tenant machine, or
> full-machine VM + full offload. In case if there is a cpu errata, KVM
> can force vmexit and dynamically intercept the selectors on all vcpus
> with filters checked. It is not supported in current RFC, but maybe
> doable in later versions.
> 
> With the above, I wonder if we can still use passthrough PMU for
> simplicity? But no strong opinion if you really want to keep this name.
> I would have to take some time to convince myself.
> 

One propoal. Maybe "direct vPMU"? I think there would be many words that
focus on the "passthrough" side but not on the "interception/mediation"
side?

> Thanks.
> -Mingwei
> > 
> > On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > 
> > > 1. host system wide / QEMU events handling during VM running
> > >    At VM-entry, all the host perf events which use host x86 PMU will be
> > >    stopped. These events with attr.exclude_guest = 1 will be stopped here
> > >    and re-started after vm-exit. These events without attr.exclude_guest=1
> > >    will be in error state, and they cannot recovery into active state even
> > >    if the guest stops running. This impacts host perf a lot and request
> > >    host system wide perf events have attr.exclude_guest=1.
> > > 
> > >    This requests QEMU Process's perf event with attr.exclude_guest=1 also.
> > > 
> > >    During VM running, perf event creation for system wide and QEMU
> > >    process without attr.exclude_guest=1 fail with -EBUSY. 
> > > 
> > > 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.
> > 
> > > 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.
> > 
> > E.g. if an IPI NMI arrives before the host's PMU is loaded, confusion may ensue.
> > SVM has the luxury of running with GIF=0, but that simply isn't an option on VMX.
> > 
> > >    need x86 subsystem to reserve the dedicated kvm_pmi_vector, and we
> > >    didn't meet the skid PMI issue on modern Intel processors.
> > > 
> > > 4. per-VM passthrough mode configuration
> > >    Current RFC uses a KVM module enable_passthrough_pmu RO parameter,
> > >    it decides vPMU is passthrough mode or emulated mode at kvm module
> > >    load time.
> > >    Do we need the capability of per-VM passthrough mode configuration?
> > >    So an admin can launch some non-passthrough VM and profile these
> > >    non-passthrough VMs in host, but admin still cannot profile all
> > >    the VMs once passthrough VM existence. This means passthrough vPMU
> > >    and emulated vPMU mix on one platform, it has challenges to implement.
> > >    As the commit message in commit 0011, the main challenge is 
> > >    passthrough vPMU and emulated vPMU have different vPMU features, this
> > >    ends up with two different values for kvm_cap.supported_perf_cap, which
> > >    is initialized at module load time. To support it, more refactor is
> > >    needed.
> > 
> > I have no objection to an all-or-nothing setup.  I'd honestly love to rip out the
> > existing vPMU support entirely, but that's probably not be realistic, at least not
> > in the near future.
> > 
> > > Remain Works
> > > ===
> > > 1. To reduce passthrough vPMU overhead, optimize the PMU context switch.
> > 
> > Before this gets out of its "RFC" phase, I would at least like line of sight to
> > a more optimized switch.  I 100% agree that starting with a conservative
> > implementation is the way to go, and the kernel absolutely needs to be able to
> > profile KVM itself (and everything KVM calls into), i.e. _always_ keeping the
> > guest PMU loaded for the entirety of KVM_RUN isn't a viable option.
> > 
> > But I also don't want to get into a situation where can't figure out a clean,
> > robust way to do the optimized context switch without needing (another) massive
> > rewrite.