On Mon, Apr 07, 2025 at 03:28:13PM +0200, Cornelia Huck wrote: > On Mon, Apr 07 2025, David Hildenbrand <da...@redhat.com> wrote: > > > On 07.04.25 15:12, Halil Pasic wrote: > >> On Mon, 7 Apr 2025 04:34:29 -0400 > >> "Michael S. Tsirkin" <m...@redhat.com> wrote: > >> > >>> On Mon, Apr 07, 2025 at 10:17:10AM +0200, David Hildenbrand wrote: > >>>> On 07.04.25 09:52, Michael S. Tsirkin wrote: > >>>>> On Fri, Apr 04, 2025 at 05:39:10PM +0200, Halil Pasic wrote: > >>>>>>> > >>>>>>> Not perfect, but AFAIKS, not horrible. > >>>>>> > >>>>>> It is like it is. QEMU does queue exist if the corresponding feature > >>>>>> is offered by the device, and that is what we have to live with. > >>>>> > >>>>> I don't think we can live with this properly though. > >>>>> It means a guest that does not know about some features > >>>>> does not know where to find things. > >>>> > >>>> Please describe a real scenario, I'm missing the point. > >>> > >>> > >>> OK so. > >>> > >>> Device has VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_REPORTING > >>> Driver only knows about VIRTIO_BALLOON_F_REPORTING so > >>> it does not know what does VIRTIO_BALLOON_F_FREE_PAGE_HINT do. > >>> How does it know which vq to use for reporting? > >>> It will try to use the free page hint one. > >> > >> First, sorry for not catching up again with the discussion earlier. > >> > >> I think David's point is based on the assumption that by the time feature > >> with the feature bit N+1 is specified and allocates a queue Q, all > >> queues with indexes smaller than Q are allocated and possibly associated > >> with features that were previously specified (and probably have feature > >> bits smaller than N+1). > >> > >> I.e. that we can mandate, even if you don't want to care about other > >> optional features, you have to, because we say so, for the matter of > >> virtqueue existence. And anything in the future, you don't have to care > >> about because the queue index associated with future features is larger > >> than Q, so it does not affect our position. > >> > >> I think that argument can fall a part if: > >> * future features reference optional queues defined in the past > >> * somebody managed to introduce a limbo where a feature is reserved, and > >> they can not decide if they want a queue or not, or make the existence > >> of the queue depend on something else than a feature bit. > > > > Staring at the cross-vmm, including the adding+removing of features and > > queues that are not in the spec, I am wondering if (in a world with > > fixed virtqueues) > > > > 1) Feature bits must be reserved before used. > > > > 2) Queue indices must be reserved before used. > > > > It all smells like a problem similar to device IDs ... > > Indeed, we need a rule "reserve a feature bit/queue index before using > it, even if you do not plan to spec it properly".
Reserving feature bits is something I do my best to advocate for in all presentations I do. -- MST
