Re: [PATCH v2 1/2] qdev: add IOThreadVirtQueueMappingList property type

[Kevin Wolf <kwolf@redhat.com>]

Am 11.12.2023 um 16:32 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 18.09.2023 um 18:16 hat Stefan Hajnoczi geschrieben:
> >> virtio-blk and virtio-scsi devices will need a way to specify the
> >> mapping between IOThreads and virtqueues. At the moment all virtqueues
> >> are assigned to a single IOThread or the main loop. This single thread
> >> can be a CPU bottleneck, so it is necessary to allow finer-grained
> >> assignment to spread the load.
> >> 
> >> Introduce DEFINE_PROP_IOTHREAD_VQ_MAPPING_LIST() so devices can take a
> >> parameter that maps virtqueues to IOThreads. The command-line syntax for
> >> this new property is as follows:
> >> 
> >>   --device '{"driver":"foo","iothread-vq-mapping":[{"iothread":"iothread0","vqs":[0,1,2]},...]}'
> >> 
> >> IOThreads are specified by name and virtqueues are specified by 0-based
> >> index.
> >> 
> >> It will be common to simply assign virtqueues round-robin across a set
> >> of IOThreads. A convenient syntax that does not require specifying
> >> individual virtqueue indices is available:
> >> 
> >>   --device '{"driver":"foo","iothread-vq-mapping":[{"iothread":"iothread0"},{"iothread":"iothread1"},...]}'
> >> 
> >> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >
> > When testing this, Qing Wang noticed that "info qtree" crashes. This is
> > because the string output visitor doesn't support structs. I suppose
> > IOThreadVirtQueueMapping is the first struct type that is used in a qdev
> > property type.
> >
> > So we'll probably have to add some kind of struct support to the string
> > output visitor before we can apply this. Even if it's as stupid as just
> > printing "<struct IOThreadVirtQueueMapping>" without actually displaying
> > the value.
> 
> The string visitors have been nothing but trouble.
> 
> For input, we can now use keyval_parse() and the QObject input visitor
> instead.  Comes with restrictions, but I'd argue it's a more solid base
> than the string input visitor.
> 
> Perhaps we can do something similar for output: create a suitable
> formatter for use it with the QObject output visitor, replacing the
> string output visitor.

dump_qobject() frm block/qapi.c?

But I wouldn't like to block this series for a major rework of "info
qtree", so I think we need to make sure that the string input visitor
doesn't crash at least.

Kevin