Re: [PATCH v7 07/13] monitor: Make current monitor a per-coroutine property

[Kevin Wolf <kwolf@redhat.com>]

Am 28.09.2020 um 09:47 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 14.09.2020 um 17:11 hat Markus Armbruster geschrieben:
> >> Kevin Wolf <kwolf@redhat.com> writes:
> >> 
> >> > This way, a monitor command handler will still be able to access the
> >> > current monitor, but when it yields, all other code code will correctly
> >> > get NULL from monitor_cur().
> >> >
> >> > This uses a hash table to map the coroutine pointer to the current
> >> > monitor of that coroutine.  Outside of coroutine context, we associate
> >> > the current monitor with the leader coroutine of the current thread.
> >> 
> >> In qemu-system-FOO, the hash table can have only these entries:
> >> 
> >> * (OOB) One mapping @mon_iothread's thread leader to a QMP monitor, while
> >>   executing a QMP command out-of-band.
> >> 
> >> * (QMP-CO) One mapping @qmp_dispatcher_co (a coroutine in the main
> >>   thread) to a QMP monitor, while executing a QMP command in-band and in
> >>   coroutine context.
> >> 
> >> * (QMP) One mapping the main thread's leader to a QMP monitor, while
> >>   executing a QMP command in-band and out of coroutine context, in a
> >>   bottom half.
> >> 
> >> * (HMP) One mapping the main thread's leader to an HMP monitor, while
> >>   executing an HMP command out of coroutine context.
> >> 
> >> * (HMP-CO) One mapping a transient coroutine in the main thread to an
> >>   HMP monitor, while executing an HMP command in coroutine context.
> >> 
> >> In-band execution is one command after the other.
> >> 
> >> Therefore, at most one monitor command can be executing in-band at any
> >> time.
> >> 
> >> Therefore, the hash table has at most *two* entries: one (OOB), and one
> >> of the other four.
> >> 
> >> Can you shoot any holes into my argument?
> >
> > I think with human-monitor-command, you can have three mappings:
> >
> > 1. The main thread's leader (it is a non-coroutine QMP command) to the
> >    QMP monitor
> 
> This is (QMP).
> 
> > 2. With a coroutine HMP command, one mapping from the transient HMP
> >    coroutine to the transient HMP monitor (with a non-coroutine HMP
> >    command, we'd instead temporarily change the mapping from 1.)
> 
> This is (HMP-CO).
> 
> > 3. The OOB entry
> 
> This is (OOB).
> 
> To get 1. (QMP) and 2, (HMP-CO) at the same time, the in-band,
> non-coroutine QMP command needs to execute interleaved with the in-band,
> coroutine HMP command.
> 
> Such an interleaving contradicts "In-band execution is one command after
> the other", which is a fundamental assumption in-band commands may make.
> If the assumption is invalid, we got a problem.  Is it?

Interleaving, or rather executing another command in the middle of its
implementation is the very purpose of human-monitor-command (which is
what I was talking about, so "the in-band non-coroutine QMP command" is
a very specific one).

It's the only command I can think of that is exceptional in this way
and would lead to three mappings.

Kevin