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Tsirkin" To: David Hildenbrand Cc: Halil Pasic , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.linux.dev, kvm@vger.kernel.org, Chandra Merla , Stable@vger.kernel.org, Cornelia Huck , Thomas Huth , Eric Farman , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Wei Wang Subject: Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues Message-ID: <20250407042058-mutt-send-email-mst@kernel.org> References: <20250404063619.0fa60a41.pasic@linux.ibm.com> <4a33daa3-7415-411e-a491-07635e3cfdc4@redhat.com> <20250404153620.04d2df05.pasic@linux.ibm.com> <20250404160025.3ab56f60.pasic@linux.ibm.com> <6f548b8b-8c6e-4221-a5d5-8e7a9013f9c3@redhat.com> <20250404173910.6581706a.pasic@linux.ibm.com> <20250407034901-mutt-send-email-mst@kernel.org> <2b187710-329d-4d36-b2e7-158709ea60d6@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2b187710-329d-4d36-b2e7-158709ea60d6@redhat.com> 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. > Whoever adds new feat_X *must be aware* about all previous features, > otherwise we'd be reusing feature bits and everything falls to pieces. The knowledge is supposed be limited to which feature bit to use. > > > > So now, I am inclined to add linux code to work with current qemu and > > with spec compliant one, and add qemu code to work with current linux > > and spec compliant one. > > > > Document the bug in the spec, maybe, in a non conformance section. > > I'm afraid this results in a lot of churn without really making things > better. > IMHO, documenting things how they actually behave, and maybe moving towards > fixed queue indexes for new features is the low hanging fruit. I worry about how to we ensure that? If old code is messed up people will just keep propagating that. I would like to fix old code so that new code is correct. > > As raised, it's not just qemu+linux, it's *at least* also cloud-hypervisor. > > -- > Cheers, > > David / dhildenb There's a slippery slope here in that people will come to us with buggy devices and ask to change the spec. -- MST