Hi Paolo, On Fri, Mar 12, 2021 at 12:11 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > 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. It makes sense. > > >> 4 bytes flags (always zero) Could you give some potential use for this flag? > >> 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) Potential use for this type? Should we move this outside descriptor? Since all stats probably have the same size. > >> - 4 bytes for the flags (for now always zero) Potential use for this flag? > >> - 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. Good point! I'll be working on the new fd-based interface and come back with new patchset. > > Paolo > Thanks, Jing
