From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4603B3F4DF7 for ; Thu, 28 May 2026 13:36:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779975403; cv=none; b=iik1cbkIm5WbOo/73+Oh9xHPatuWswhPxT9OsiQQHMAqUGLxkApUbh4Xw36FeQGdCv/vhHDVN4WFA/89PqoQyo9JhHd+OjAKR+P/Ri8YdOmsUZdWezLt+J/WxzN+sb+zmQyCslNLj6yDKP9aPhevwtHL/0DiLsXX8cdMsoaiHZc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779975403; c=relaxed/simple; bh=8aqO8+G9DRd2d5B2Yho6kewIVD3sY18FqplaMcnlEdQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=X22R9ubt/dxuRsF4e/74Nxl+ZFTJKqBemyBvMOLNWT7g5Xd4ZX1eYNUf8VxiWUBYaOk5cR6WPfe9ECsvBH35c52rwVkFqeA7cFrpHiWfm0GiF6dV7AUaAwTN/iVQOci9WsVD0jf5xhaUpa2fgadAY2AhlSrhi9b3L6PL6adV48U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=CJL53iJE; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="CJL53iJE" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1779975401; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pL78MkQJHoHepSh4XUcbnvy159epezgo6oVkPfuLm0U=; b=CJL53iJEm8elVHN+8urbnpOywpqqpqh8bZcZ5jNbK1ajPCCC496dYeAZZuQ9U9cbuMnaaU ep7p6l669sJBapWtIPbNJUTq2d/idS/pZQ85Tiz1QVJxsTI6U+l61lQ56zgMkg1FeNKyE8 8MuI5Irt7odTZuJSHvkuQnwkXTezyts= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-245-P-J7UxovPlqlSrQdJqDxYQ-1; Thu, 28 May 2026 09:36:37 -0400 X-MC-Unique: P-J7UxovPlqlSrQdJqDxYQ-1 X-Mimecast-MFC-AGG-ID: P-J7UxovPlqlSrQdJqDxYQ_1779975395 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8502A18005B9; Thu, 28 May 2026 13:36:35 +0000 (UTC) Received: from localhost (unknown [10.2.16.62]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 2925418004A3; Thu, 28 May 2026 13:36:34 +0000 (UTC) Date: Thu, 28 May 2026 09:36:33 -0400 From: Stefan Hajnoczi To: Manos Pitsidianakis Cc: Parav Pandit , Demi Marie Obenour , "virtio-comment@lists.linux.dev" Subject: Re: MSI-X vector limits and reserving a virtio device ID Message-ID: <20260528133633.GB40266@fedora> References: <11771164-7919-43e1-a980-03f036bdae2e@gmail.com> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="GIkM2LLcpkUHVxTN" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 --GIkM2LLcpkUHVxTN Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, May 28, 2026 at 08:47:25AM +0300, Manos Pitsidianakis wrote: > On Thu, May 28, 2026 at 8:22=E2=80=AFAM Parav Pandit w= rote: > > > > > > > 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. >=20 > This exactly. >=20 > Furthermore, we already have a mechanism for "providing" an arbitrary > virtio device; it's called a transport. >=20 > Demi, I suggest you look into virtio-msg transport, which would allow > you to do what you want. I think the "virtio vhost-guest" device is a continuation of the virtio-vhost-user device work: https://wiki.qemu.org/Features/VirtioVhostUser It is a VIRTIO device type, not a transport. The idea is that a driver VM runs a normal VIRTIO driver (e.g. virtio-net-pci) and a second device VM runs the virtio-net device implementation. It's similar to running a vhost-user-net device on the host, except now the device implementation runs inside a VM instead of a process on the host. The virtio-vhost-user device is what the device VM sees and uses to provide the virtio-net device to the driver VM. Stefan >=20 > > > > > Also, I'd like to know which systems have a small limit on the number > > > of MSI-X vectors. > > I didn=E2=80=99t 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 =3D 775 of them! That c= an, > > > 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. > > > > > -- > > > Sincerely, > > > Demi Marie Obenour (she/her/hers) >=20 > --=20 > Manos Pitsidianakis > Emulation and Virtualization Engineer at Linaro Ltd >=20 --GIkM2LLcpkUHVxTN Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmoYROEACgkQnKSrs4Gr c8iY7gf+OFlvve9iwwDpHhFNW2SxEEC73JkyDY90aaFjDgEXlMBk7OvdERn93sQg 3bdPwe2+C0ncYmiaLhvU6C9TcsgjVF5uWfFl7h1NliDN4LA9cGEOj13SyUMKSYHI JeFZgZefhTOua4XILjmWeTJSXgNngY5XhnRyiyTd3a6vJn8s0zb8TY8CaEg/kfEL Er7rNGYHLTrxZFqpTk88HSo5BMNFZtFoeT//hz5S4ojvnGyaHGDccJaKFgGs+p2B ub4gGoCbuA99qtH+7giNKlttXWblCWVULMUclf6qwNuuXohCH3WYHnU5MfJf/y16 pYFou3RX2g2Vvr2Fo9SKoSr2UklVnQ== =wcFT -----END PGP SIGNATURE----- --GIkM2LLcpkUHVxTN--