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(p200300cbc7383c008b014fd9b833e1e9.dip0.t-ipconnect.de. [2003:cb:c738:3c00:8b01:4fd9:b833:e1e9]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43ec36699e0sm122875385e9.35.2025.04.07.01.44.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Apr 2025 01:44:24 -0700 (PDT) Message-ID: <0c221abf-de20-4ce3-917d-0375c1ec9140@redhat.com> Date: Mon, 7 Apr 2025 10:44:21 +0200 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues To: "Michael S. Tsirkin" 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 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> <20250407042058-mutt-send-email-mst@kernel.org> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <20250407042058-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.04.25 10:34, Michael S. Tsirkin 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. "only knows" -- VIRTIO_BALLOON_F_FREE_PAGE_HINT was proposed + specified in the spec. So I think this is not a very good example. > > > >> 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. I think we also have to know which virtqueue bits can be used, right? I mean, I agree that it's all nasty ... >>> >>> 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. I yet have to fully digest cross-vm: https://github.com/google/crosvm/blob/main/devices/src/virtio/balloon.rs and how it would free-page-reporting work with upstream Linux ... :( -- Cheers, David / dhildenb