Re: [PATCH kvm/queue v2 2/3] perf: x86/core: Add interface to query perfmon_event_map[] directly
From: Liang, Kan
Date: Thu Feb 10 2022 - 14:46:18 EST
On 2/10/2022 2:16 PM, Jim Mattson wrote:
On Thu, Feb 10, 2022 at 10:30 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
On 2/10/2022 11:34 AM, Jim Mattson wrote:
On Thu, Feb 10, 2022 at 7:34 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
On 2/9/2022 2:24 PM, David Dunn wrote:
Dave,
In my opinion, the right policy depends on what the host owner and
guest owner are trying to achieve.
If the PMU is being used to locate places where performance could be
improved in the system, there are two sub scenarios:
- The host and guest are owned by same entity that is optimizing
overall system. In this case, the guest doesn't need PMU access and
better information is provided by profiling the entire system from the
host.
- The host and guest are owned by different entities. In this
case, profiling from the host can identify perf issues in the guest.
But what action can be taken? The host entity must communicate issues
back to the guest owner through some sort of out-of-band information
channel. On the other hand, preempting the host PMU to give the guest
a fully functional PMU serves this use case well.
TDX and SGX (outside of debug mode) strongly assume different
entities. And Intel is doing this to reduce insight of the host into
guest operations. So in my opinion, preemption makes sense.
There are also scenarios where the host owner is trying to identify
systemwide impacts of guest actions. For example, detecting memory
bandwidth consumption or split locks. In this case, host control
without preemption is necessary.
To address these various scenarios, it seems like the host needs to be
able to have policy control on whether it is willing to have the PMU
preempted by the guest.
But I don't see what scenario is well served by the current situation
in KVM. Currently the guest will either be told it has no PMU (which
is fine) or that it has full control of a PMU. If the guest is told
it has full control of the PMU, it actually doesn't. But instead of
losing counters on well defined events (from the guest perspective),
they simply stop counting depending on what the host is doing with the
PMU.
For the current perf subsystem, a PMU should be shared among different
users via the multiplexing mechanism if the resource is limited. No one
has full control of a PMU for lifetime. A user can only have the PMU in
its given period. I think the user can understand how long it runs via
total_time_enabled and total_time_running.
For most clients, yes. For kvm, no. KVM currently tosses
total_time_enabled and total_time_running in the bitbucket. It could
extrapolate, but that would result in loss of precision. Some guest
uses of the PMU would not be able to cope (e.g.
https://github.com/rr-debugger/rr).
For a guest, it should rely on the host to tell whether the PMU resource
is available. But unfortunately, I don't think we have such a
notification mechanism in KVM. The guest has the wrong impression that
the guest can have full control of the PMU.
That is the only impression that the architectural specification
allows the guest to have. On Intel, we can mask off individual fixed
counters, and we can reduce the number of GP counters, but AMD offers
us no such freedom. Whatever resources we advertise to the guest must
be available for its use whenever it wants. Otherwise, PMU
virtualization is simply broken.
In my opinion, we should add the notification mechanism in KVM. When the
PMU resource is limited, the guest can know whether it's multiplexing or
can choose to reschedule the event.
That sounds like a paravirtual perf mechanism, rather than PMU
virtualization. Are you suggesting that we not try to virtualize the
PMU? Unfortunately, PMU virtualization is what we have customers
clamoring for. No one is interested in a paravirtual perf mechanism.
For example, when will VTune in the guest know how to use your
proposed paravirtual interface?
OK. If KVM cannot notify the guest, maybe guest can query the usage of
counters before using a counter. There is a IA32_PERF_GLOBAL_INUSE MSR
introduced with Arch perfmon v4. The MSR provides an "InUse" bit for
each counters. But it cannot guarantee that the counter can always be
owned by the guest unless the host treats the guest as a super-user and
agrees to not touch its counter. This should only works for the Intel
platforms.
Simple question: Do all existing guests (Windows and Linux are my
primary interest) query that MSR today? If not, then this proposal is
DOA.
No, we don't, at least for Linux. Because the host own everything. It
doesn't need the MSR to tell which one is in use. We track it in an SW way.
For the new request from the guest to own a counter, I guess maybe it is
worth implementing it. But yes, the existing/legacy guest never check
the MSR.
But seems the notification mechanism may not work for TDX case?
On the other hand, if we flip it around the semantics are more clear.
A guest will be told it has no PMU (which is fine) or that it has full
control of the PMU. If the guest is told that it has full control of
the PMU, it does. And the host (which is the thing that granted the
full PMU to the guest) knows that events inside the guest are not
being measured. This results in all entities seeing something that
can be reasoned about from their perspective.
I assume that this is for the TDX case (where the notification mechanism
doesn't work). The host still control all the PMU resources. The TDX
guest is treated as a super-user who can 'own' a PMU. The admin in the
host can configure/change the owned PMUs of the TDX. Personally, I think
it makes sense. But please keep in mind that the counters are not
identical. There are some special events that can only run on a specific
counter. If the special counter is assigned to TDX, other entities can
never run some events. We should let other entities know if it happens.
Or we should never let non-host entities own the special counter.
Right; the counters are not fungible. Ideally, when the guest requests
a particular counter, that is the counter it gets. If it is given a
different counter, the counter it is given must provide the same
behavior as the requested counter for the event in question.
Ideally, Yes, but sometimes KVM/host may not know whether they can use
another counter to replace the requested counter, because KVM/host
cannot retrieve the event constraint information from guest.
In that case, don't do it. When the guest asks for a specific counter,
give the guest that counter. This isn't rocket science.
Sounds like the guest can own everything if they want. Maybe it makes
sense from the virtualization's perspective. But it sounds too
aggressive to me. :)
Thanks,
Kan
For example, we have Precise Distribution (PDist) feature enabled only
for the GP counter 0 on SPR. Perf uses the precise_level 3 (a SW
variable) to indicate the feature. For the KVM/host, they never know
whether the guest apply the PDist feature.
I have a patch that forces the perf scheduler starts from the regular
counters, which may mitigates the issue, but cannot fix it. (I will post
the patch separately.)
Or we should never let the guest own the special counters. Although the
guest has to lose some special events, I guess the host may more likely
be willing to let the guest own a regular counter.
Thanks,
Kan
Thanks,
Kan
Thanks,
Dave Dunn
On Wed, Feb 9, 2022 at 10:57 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
I was referring to gaps in the collection of data that the host perf
subsystem doesn't know about if ATTRIBUTES.PERFMON is set for a TDX
guest. This can potentially be a problem if someone is trying to
measure events per unit of time.
Ahh, that makes sense.
Does SGX cause problem for these people? It can create some of the same
collection gaps:
performance monitoring activities are suppressed when entering
an opt-out (of performance monitoring) enclave.