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

[Markus Armbruster <armbru@redhat.com>]

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.