Re: [PATCH v14 08/11] qapi/s390/cpu topology: change-topology monitor command

[Kevin Wolf]

Am 18.01.2023 um 11:53 hat Thomas Huth geschrieben:
> On 17/01/2023 14.31, Nina Schoetterl-Glausch wrote:
> > On Tue, 2023-01-17 at 08:30 +0100, Thomas Huth wrote:
> > > On 16/01/2023 22.09, Nina Schoetterl-Glausch wrote:
> > > > On Thu, 2023-01-05 at 15:53 +0100, Pierre Morel wrote:
> > > > > The modification of the CPU attributes are done through a monitor
> > > > > commands.
> > > > > 
> > > > > It allows to move the core inside the topology tree to optimise
> > > > > the cache usage in the case the host's hypervizor previously
> > > > > moved the CPU.
> > > > > 
> > > > > The same command allows to modifiy the CPU attributes modifiers
> > > > > like polarization entitlement and the dedicated attribute to notify
> > > > > the guest if the host admin modified scheduling or dedication of a 
> > > > > vCPU.
> > > > > 
> > > > > With this knowledge the guest has the possibility to optimize the
> > > > > usage of the vCPUs.
> > > > > 
> > > > > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > > > > ---
> ...
> > > > > +    s390_topology.sockets[s390_socket_nb(id)]--;
> > > > 
> > > > I suppose this function cannot run concurrently, so the same CPU 
> > > > doesn't get removed twice.
> > > 
> > > QEMU has the so-called BQL - the Big Qemu Lock. Instructions handlers are
> > > normally called with the lock taken, see qemu_mutex_lock_iothread() in
> > > target/s390x/kvm/kvm.c.
> > 
> > That is good to know, but is that the relevant lock here?
> > We don't want to concurrent qmp commands. I looked at the code and it's 
> > pretty complicated.
> 
> Not sure, but I believe that QMP commands are executed from the main
> iothread, so I think this should be safe? ... CC:-ing some more people who
> might know the correct answer.

In general yes, QMP commands are processed one after another in the main
thread while holding the BQL. And I think this is the relevant case for
you.

The exception is out-of-band commands, which I think run in the monitor
thread while some other (normal) command could be processed. OOB
commands are quite limited in what they are allowed to do, though, and
there aren't many of them. They are mainly meant to fix situations where
something (including other QMP commands) got stuck.

Kevin