From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5B53DC636D3 for ; Wed, 1 Feb 2023 10:57:20 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pNAlD-0007gs-U5; Wed, 01 Feb 2023 05:54:40 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pNAkJ-00077j-2s for qemu-devel@nongnu.org; Wed, 01 Feb 2023 05:53:49 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pNAkE-0008Q1-J5 for qemu-devel@nongnu.org; Wed, 01 Feb 2023 05:53:40 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1675248817; 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=AL2Pdi3is1+oRvaU0I7CYlC6ByOWz3/8VnYI5SjBaV8=; b=UIeK4k0FQKTfYNYTz9BFukkYuXHHvcdrlmrlzrP/ldnRKyTVUsUhghWMmxAUZam2S3q4kH OBVSFyd5LAAjCQvyWgokWXlGNQi9a4w+kmEX41BgW618EmLVnBME2Ak2fLq65PRbv7qcw1 JCLfFFktgfyGugp/pbmVbI8uzMAx8Bw= Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-255-duhcWVHMOg-5N-Q0UT94qg-1; Wed, 01 Feb 2023 05:53:36 -0500 X-MC-Unique: duhcWVHMOg-5N-Q0UT94qg-1 Received: by mail-wr1-f70.google.com with SMTP id i9-20020a0560001ac900b002bfda39265aso2005199wry.13 for ; Wed, 01 Feb 2023 02:53:36 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=AL2Pdi3is1+oRvaU0I7CYlC6ByOWz3/8VnYI5SjBaV8=; b=eW3abIqfAyagQbeMd+857GuvdEw69LsbrckabAIcXzHbpBRvGgSHSibn1ZNCrIxjeQ QU6Vxtl+/G4AIIytMRgu4YhyHSlGF+MfM9YlKwbTngXnM72t4gRMClrwaaQ9fTGZRzmN 4YKjySKf4gET34w4/Da1RmcabePJMFDbNCx93CBACABZ1aUBHwDqWFc0hZxw1zUhvGI5 PeHrU2YJHQptPofRvLe20XFHOQN/aGG6sWxLa+iUXMjC4f/FNGKnBA3hSQkcjDrSZKsw LzVLoPQ/vAyd9fNwh0DwztVViVpf21TxnRUKwpKoDU+8CTn2x+k9a+LvE21QX8z7jw03 9w7g== X-Gm-Message-State: AO0yUKVggHTdI14+G9qoqb3nAPTTbqVUztrLEWsuNT9KSkITRY7FhSvp sH/qLRCyfmqLz3FFvtObHulCI5wjYaNGvq1vayjh7Loqw26DrkC1KakfoWjpvOqVxX0OxnUHnHP q12akUGnUHBIIr8A= X-Received: by 2002:a05:600c:204e:b0:3db:66e:cfdd with SMTP id p14-20020a05600c204e00b003db066ecfddmr1697106wmg.9.1675248815185; Wed, 01 Feb 2023 02:53:35 -0800 (PST) X-Google-Smtp-Source: AK7set8D6iUCIx8+RP51BGkF3pB8weuoz2j9j4bgec8TzUBaMUsWhv0WmH0HOmkoZWe4wZULagb69A== X-Received: by 2002:a05:600c:204e:b0:3db:66e:cfdd with SMTP id p14-20020a05600c204e00b003db066ecfddmr1697087wmg.9.1675248814958; Wed, 01 Feb 2023 02:53:34 -0800 (PST) Received: from redhat.com ([2.52.144.173]) by smtp.gmail.com with ESMTPSA id j19-20020a05600c42d300b003dc53217e07sm1281147wme.16.2023.02.01.02.53.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Feb 2023 02:53:34 -0800 (PST) Date: Wed, 1 Feb 2023 05:53:31 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: Eugenio Perez Martin , Maxime Coquelin , Cindy Lu , Stefano Garzarella , qemu-level , Laurent Vivier , Juan Quintela Subject: Re: Emulating device configuration / max_virtqueue_pairs in vhost-vdpa and vhost-user Message-ID: <20230201055236-mutt-send-email-mst@kernel.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Feb 01, 2023 at 11:29:35AM +0800, Jason Wang wrote: > On Wed, Feb 1, 2023 at 3:11 AM Eugenio Perez Martin wrote: > > > > On Tue, Jan 31, 2023 at 8:10 PM Eugenio Perez Martin > > wrote: > > > > > > Hi, > > > > > > The current approach of offering an emulated CVQ to the guest and map > > > the commands to vhost-user is not scaling well: > > > * Some devices already offer it, so the transformation is redundant. > > > * There is no support for commands with variable length (RSS?) > > > > > > We can solve both of them by offering it through vhost-user the same > > > way as vhost-vdpa do. With this approach qemu needs to track the > > > commands, for similar reasons as vhost-vdpa: qemu needs to track the > > > device status for live migration. vhost-user should use the same SVQ > > > code for this, so we avoid duplications. > > > > > > One of the challenges here is to know what virtqueue to shadow / > > > isolate. The vhost-user device may not have the same queues as the > > > device frontend: > > > * The first depends on the actual vhost-user device, and qemu fetches > > > it with VHOST_USER_GET_QUEUE_NUM at the moment. > > > * The qemu device frontend's is set by netdev queues= cmdline parameter in qemu > > > > > > For the device, the CVQ is the last one it offers, but for the guest > > > it is the last one offered in config space. > > > > > > To create a new vhost-user command to decrease that maximum number of > > > queues may be an option. But we can do it without adding more > > > commands, remapping the CVQ index at virtqueue setup. I think it > > > should be doable using (struct vhost_dev).vq_index and maybe a few > > > adjustments here and there. > > > > > > Thoughts? > > > > > > Thanks! > > > > > > (Starting a separated thread to vhost-vdpa related use case) > > > > This could also work for vhost-vdpa if we ever decide to honor netdev > > queues argument. It is totally ignored now, as opposed to the rest of > > backends: > > * vhost-kernel, whose tap device has the requested number of queues. > > * vhost-user, that errors with ("you are asking more queues than > > supported") if the vhost-user parent device has less queues than > > requested (by vhost-user msg VHOST_USER_GET_QUEUE_NUM). > > > > One of the reasons for this is that device configuration space is > > totally passthrough, with the values for mtu, rss conditions, etc. > > This is not ideal, as qemu cannot check src and destination > > equivalence and they can change under the feets of the guest in the > > event of a migration. > > This looks not the responsibility of qemu but the upper layer (to > provision the same config/features in src/dst). > > > External tools are needed for this, duplicating > > part of the effort. > > > > Start intercepting config space accesses and offering an emulated one > > to the guest with this kind of adjustments is beneficial, as it makes > > vhost-vdpa more similar to the rest of backends, making the surprise > > on a change way lower. > > This probably needs more thought, since vDPA already provides a kind > of emulation in the kernel. My understanding is that it would be > sufficient to add checks to make sure the config that guests see is > consistent with what host provisioned? > > Thanks I feel ability to adjust and emulate one device on top of another one is a strong suit of virtio (as opposed to e.g. vfio which always wants exactly the same device). Let's not throw this baby out with the bathwater. > > > > Thoughts? > > > > Thanks! > >