On 15.01.20 14:43, Milan Pandurov wrote:
KVM exposes debug counters through individual debugfs files.
Monitoring these counters requires debugfs to be enabled/accessible for
the application, which might not be always the case.
Additionally, periodic monitoring multiple debugfs files from
userspace requires multiple file open/read/close + atoi conversion
operations, which is not very efficient.
Let's expose new interface to userspace for garhering these
statistics with one ioctl.
Two new ioctl methods are added:
- KVM_GET_SUPPORTED_DEBUGFS_STAT : Returns list of available counter
names. Names correspond to the debugfs file names
- KVM_GET_DEBUGFS_VALUES : Returns list of u64 values each
corresponding to a value described in KVM_GET_SUPPORTED_DEBUGFS_STAT.
Userspace application can read counter description once using
KVM_GET_SUPPORTED_DEBUGFS_STAT and periodically invoke the
KVM_GET_DEBUGFS_VALUES to get value update.
Signed-off-by: Milan Pandurov <milanpa@xxxxxxxxx>
Thanks a lot! I really love the idea to get stats directly from the kvm
vm fd owner. This is so much better than poking at files from a random
unrelated debug or stat file system.
I have a few comments overall though:
1)
This is an interface that requires a lot of logic and buffers from user
space to retrieve individual, explicit counters. What if I just wanted
to monitor the number of exits on every user space exit?
Also, we're suddenly making the debugfs names a full ABI, because
through this interface we only identify the individual stats through
their names. That means we can not remove stats or change their names,
because people may rely on them, no? Thining about this again, maybe
they already are an ABI because people rely on them in debugfs though?
I see two alternatives to this approach here:
a) ONE_REG
We can just add a new DEBUG arch in ONE_REG and expose ONE_REG per VM as
well (if we really have to?). That gives us explicit identifiers for
each stat with an explicit path to introduce new ones with very unique
identifiers.
That would give us a very easily structured approach to retrieve
individual counters.
b) part of the mmap'ed vcpu struct
We could simply move the counters into the shared struct that we expose
to user space via mmap. IIRC we only have that per-vcpu, but then again
most counters are per-vcpu anyway, so we would push the aggregation to
user space.
For per-vm ones, maybe we can just add another mmap'ed shared page for
the vm fd?
2) vcpu counters
Most of the counters count on vcpu granularity, but debugfs only gives
us a full VM view. Whatever we do to improve the situation, we should
definitely try to build something that allows us to get the counters per
vcpu (as well).
The main purpose of these counters is monitoring. It can be quite
important to know that only a single vCPU is going wild, compared to all
of them for example.
Thanks,
Alex
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