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

[Kevin Wolf]

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

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.)

3. The OOB entry

> I suspect there's a simpler solution struggling to get out.  But this
> solution works, so in it goes.  Should the simpler one succeed at
> getting out, it can go in on top.  If not, I'll probably add even more
> comments to remind myself of these facts.

I think the simple approach you had posted could work if you can fill in
the HMP part, but I think it wasn't completely trivial and you told me
not to bother for now, so I didn't. It may still be a viable path
forward if you like it better.

Kevin