On Thu, Sep 7, 2017 at 10:18 AM, Dr. David Alan Gilbert <dgilb...@redhat.com> 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. >> >> 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.
In what circumstances is the pc useful? If the client just wants the vcpu -> thread ID mapping, it doesn't matter at all. If the CPU is halted, then the PC is already accurate and synchronization isn't a problem. If the CPU is running, then an accurate PC is meaningless since it will have changed the moment the monitor command completes. We might as well just keep a copy of the last PC when entering QEMU in a vcpu thread. So I think we can offer a perfectly useful PC value without syncing. Stefan
