Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/03/21 22:41, Jing Zhang wrote:
>> I would prefer a completely different interface, where you have a file
>> descriptor that can be created and associated to a vCPU or VM (or even
>> to /dev/kvm).  Having a file descriptor is important because the fd can
>> be passed to a less-privileged process that takes care of gathering the
>> metrics
> Separate file descriptor solution is very tempting. We are still considering it
> seriously. Our biggest concern is that the metrics gathering/handling process
> is not necessary running on the same node as the one file descriptor belongs to.
> It scales better to pass metrics data directly than to pass file descriptors.

If you want to pass metrics data directly, you can just read the file 
descriptor from your VMM, just like you're using the ioctls now. 
However the file descriptor also allows a privilege-separated same-host 
interface.

>> 4 bytes flags (always zero)
>> 4 bytes number of statistics
>> 4 bytes offset of the first stat description
>> 4 bytes offset of the first stat value
>> stat descriptions:
>>    - 4 bytes for the type (for now always zero: uint64_t)
>>    - 4 bytes for the flags (for now always zero)
>>    - length of name
>>    - name
>> statistics in 64-bit format
> 
> The binary format presented above is very flexible. I understand why it is
> organized this way.
> In our situation, the metrics data could be pulled periodically as short as
> half second. They are used by different kinds of monitors/triggers/alerts.
> To enhance efficiency and reduce traffic caused by metrics passing, we
> treat all metrics info/data as two kinds. One is immutable information,
> which doesn't change in a given system boot. The other is mutable
> data (statistics data), which is pulled/transferred periodically at a high
> frequency.

The format allows to place the values before the descriptions.  So you 
could use pread to only read the first part of the file descriptor, and 
the file_operations implementation would then skip the work of building 
the immutable data.  It doesn't have to be implemented from the
beginning like that, but the above format supports it.

Paolo