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

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/03/21 23:31, Jing Zhang wrote:
> We are considering about how to create the file descriptor. It might be risky
> to create an extra fd for every vCPU. It will easily hit the fd limit for the
> process or the system for machines running a ton of small VMs.

You already have a file descriptor for every vCPU, but I agree that 
having twice as many is not very good.

> Looks like creating an extra file descriptor for every VM is a better option.
> And then we can check per vCPU stats through Ioctl of this VM fd by
> passing the vCPU index.

The file descriptor idea is not really infeasible I think (not just 
because the # of file descriptors is "only" doubled, but also because 
most of the time I think you'd only care of per-VM stats).

If you really believe it's not usable for you, you can use two ioctls to 
fill the description and the data respectively (i.e. ioctl(fd, 
KVM_GET_STATS_{DESCRIPTION,VALUES}, pdata) using the same layout as 
below.  If called with NULL argument, the ioctl returns how much data 
they will fill in.

The (always zero) global flags can be replaced by the value returned by 
KVM_CHECK_EXTENSION.

The number of statistics can be obtained by ioctl(fd, 
KVM_GET_STATS_VALUES, NULL), just divide the returned value by 8.

Paolo

>> 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