On Thu, Sep 07, 2017 at 10:18:16AM +0100, Dr. David Alan Gilbert wrote: > * Stefan Hajnoczi (stefa...@gmail.com) wrote: > > On Thu, Sep 7, 2017 at 9:13 AM, Peter Xu <pet...@redhat.com> wrote: > > > On Wed, Sep 06, 2017 at 12:54:28PM +0100, Daniel P. Berrange wrote: > > >> On Wed, Sep 06, 2017 at 12:31:58PM +0100, Dr. David Alan Gilbert wrote: > > >> > * Daniel P. Berrange (berra...@redhat.com) wrote: > > >> > > This does imply that you need a separate monitor I/O processing, > > >> > > from the > > >> > > command execution thread, but I see no need for all commands to > > >> > > suddenly > > >> > > become async. Just allowing interleaved replies is sufficient from > > >> > > the > > >> > > POV of the protocol definition. This interleaving is easy to handle > > >> > > from > > >> > > the client POV - just requires a unique 'serial' in the request by > > >> > > the > > >> > > client, that is copied into the reply by QEMU. > > >> > > > >> > OK, so for that we can just take Marc-André's syntax and call it 'id': > > >> > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.html > > >> > > > >> > then it's upto the caller to ensure those id's are unique. > > >> > > >> Libvirt has in fact generated a unique 'id' for every monitor command > > >> since day 1 of supporting QMP. > > >> > > >> > I do worry about two things: > > >> > a) With this the caller doesn't really know which commands could be > > >> > in parallel - for example if we've got a recovery command that's > > >> > executed by this non-locking thread that's OK, we expect that > > >> > to be doable in parallel. If in the future though we do > > >> > what you initially suggested and have a bunch of commands get > > >> > routed to the migration thread (say) then those would suddenly > > >> > operate in parallel with other commands that we're previously > > >> > synchronous. > > >> > > >> We could still have an opt-in for async commands. eg default to executing > > >> all commands in the main thread, unless the client issues an explicit > > >> "make it async" command, to switch to allowing the migration thread to > > >> process it async. > > >> > > >> { "execute": "qmp_allow_async", > > >> "data": { "commands": [ > > >> "migrate_cancel", > > >> ] } } > > >> > > >> > > >> { "return": { "commands": [ > > >> "migrate_cancel", > > >> ] } } > > >> > > >> The server response contains the subset of commands from the request > > >> for which async is supported. > > >> > > >> That gives good negotiation ability going forward as we incrementally > > >> support async on more commands. > > > > > > I think this goes back to the discussion on which design we'd like to > > > choose. IMHO the whole async idea plus the per-command-id is indeed > > > cleaner and nicer, and I believe that can benefit not only libvirt, > > > but also other QMP users. The problem is, I have no idea how long > > > it'll take to let us have such a feature - I believe that will include > > > QEMU and Libvirt to both support that. And it'll be a pity if the > > > postcopy recovery cannot work only because we cannot guarantee a > > > stable monitor. > > > > Please don't rush in a hack, they often introduce new bugs that we > > have to support long-term when they are part of the QMP API.
Sorry, I wasn't meant to push anything. I was trying to see what would be the best way to go. > > > > In your original email you mentioned "info cpus". Have you considered > > modifying this command so it does not sync the CPU? I'm not sure > > callers really need to sync the CPU, typically they just want to know > > the vcpu numbers, thread IDs, and current state (halted, running, > > etc). > > But it has the pc as well, so that's actual state. Yes. Even if we don't need to sync pc regs for this single "info cpus" command, I do feel slightly awkward if we don't allow things like syncing CPU to happen in any command. IMHO we just need to make sure these commands may block. > > Dave > > > The next step after that would be to audit other monitor commands for > > unnecessary vcpu synchronization. It's really hard to do this for every single command, at least to me. Comparing to this, I think now I more prefer what Dan has suggested in the other reply to have an extra way to request async commands while keep the rest of commands compatible (though obviously I misunderstood the email when I was writting up previous reply...). Thanks, -- Peter Xu
