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

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> 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).

Got it now, thanks.

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