On Mon, Apr 07 2025, David Hildenbrand <david@xxxxxxxxxx> wrote: > On 07.04.25 15:12, Halil Pasic wrote: >> On Mon, 7 Apr 2025 04:34:29 -0400 >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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".
