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Tsirkin" To: Demi Marie Obenour Cc: Parav Pandit , "virtio-comment@lists.linux.dev" Subject: Re: MSI-X vector limits and reserving a virtio device ID Message-ID: <20260528032227-mutt-send-email-mst@kernel.org> References: <11771164-7919-43e1-a980-03f036bdae2e@gmail.com> <0f70bfea-3344-405e-bb69-c36ab8c353bc@gmail.com> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <0f70bfea-3344-405e-bb69-c36ab8c353bc@gmail.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: qOTtkS7w3hy-54VfoeZoe4Xia20VxFHRgSavN4IQvZ0_1779952994 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit On Thu, May 28, 2026 at 03:03:00AM -0400, Demi Marie Obenour wrote: > On 5/28/26 01:22, Parav Pandit wrote: > > > >> From: Demi Marie Obenour > >> Sent: 28 May 2026 05:23 AM > >> To: virtio-comment@lists.linux.dev > >> Subject: MSI-X vector limits and reserving a virtio device ID > >> > >> I'd like to reserve a virtio device ID for virtio vhost-guest, > >> formally virtio vhost-user. Would this be possible? > >> > > Vhost user is an implementation of the device. > > I believe it stays as implementation and not a new device type. > > Virtio vhost-user is a device used to implement a vhost-user device > from another VM. > > >> Also, I'd like to know which systems have a small limit on the number > >> of MSI-X vectors. > > I didn’t follow the question. Typically, Intel cpu has a limit of 256 vectors per core. > > > >> The vhost-guest device uses lots of MSI-X vectors. > >> Typical implementations will use 256 * 3 + 7 = 775 of them! That can, > >> of course, be reduced by sharing vectors, but this is undesirable > >> as it comes at a performance cost. > > Did you get a chance to measure the performance, for which device type? > > Interrupts are generally moderated. > > In other devices (scalable functions of mlx5) I have measured perf of 8 to 16 queues sharing a vectors has < 4% perf drop. > > I'm more concerned with whether it is a problem to assume that > 775 MSI-X interrupts can be exposed by a virtual device. Why not? But many hosts will have to share them. > -- > Sincerely, > Demi Marie Obenour (she/her/hers)