From patchwork Wed Jul 12 09:17:04 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Hanna Czenczek X-Patchwork-Id: 13309882 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 B5349EB64D9 for ; Wed, 12 Jul 2023 09:17:42 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qJVyD-0007pc-8Q; Wed, 12 Jul 2023 05:17:13 -0400 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 1qJVyB-0007o7-U7 for qemu-devel@nongnu.org; Wed, 12 Jul 2023 05:17:12 -0400 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 1qJVyA-0003aO-Cb for qemu-devel@nongnu.org; Wed, 12 Jul 2023 05:17:11 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1689153427; 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: content-transfer-encoding:content-transfer-encoding; bh=w/kBTzdJ4GcIyvsYK2oLo+LlPMQF1Cd3QatUllqac0o=; b=J9aXVueY2VHadyuDd2myJYCKiqyq0S7eSxwCg2A+dR9oVgQVI44FnYrxGj8x7oZGjgyeT2 lhxVxp3DY12ZpjnKOvFssrsHR1+lrRDK4mOpFan7loewNVfyUQGgq8V+GsfFoeIISIQ1BA dAeK0672uUbiQAV0TBgPR0EBnEzlKlY= Received: from mimecast-mx02.redhat.com (66.187.233.73 [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-575-_uDaMlx5OLeHGTU0F8UntQ-1; Wed, 12 Jul 2023 05:17:06 -0400 X-MC-Unique: _uDaMlx5OLeHGTU0F8UntQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id D1395380391A for ; Wed, 12 Jul 2023 09:17:05 +0000 (UTC) Received: from localhost (unknown [10.39.193.231]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 71BF140C2063; Wed, 12 Jul 2023 09:17:05 +0000 (UTC) From: Hanna Czenczek To: qemu-devel@nongnu.org Cc: Hanna Czenczek , "Michael S . Tsirkin" , Stefan Hajnoczi , German Maglione Subject: [PATCH] vhost-user.rst: Clarify enabling/disabling vrings Date: Wed, 12 Jul 2023 11:17:04 +0200 Message-ID: <20230712091704.15589-1-hreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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 Currently, the vhost-user documentation says that rings are to be initialized in a disabled state when VHOST_USER_F_PROTOCOL_FEATURES is negotiated. However, by the time of feature negotiation, all rings have already been initialized, so it is not entirely clear what this means. At least the vhost-user-backend Rust crate's implementation interpreted it to mean that whenever this feature is negotiated, all rings are to be put into a disabled state, which means that every SET_FEATURES call would disable all rings, effectively halting the device. This is problematic because the VHOST_F_LOG_ALL feature is also set or cleared this way, which happens during migration. Doing so should not halt the device. Other implementations have interpreted this to mean that the device is to be initialized with all rings disabled, and a subsequent SET_FEATURES call that does not set VHOST_USER_F_PROTOCOL_FEATURES will enable all of them. Here, SET_FEATURES will never disable any ring. This other interpretation does not suffer the problem of unintentionally halting the device whenever features are set or cleared, so it seems better and more reasonable. We should clarify this in the documentation. Signed-off-by: Hanna Czenczek --- docs/interop/vhost-user.rst | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst index 5a070adbc1..ca0e899765 100644 --- a/docs/interop/vhost-user.rst +++ b/docs/interop/vhost-user.rst @@ -383,12 +383,23 @@ and stop ring upon receiving ``VHOST_USER_GET_VRING_BASE``. Rings can be enabled or disabled by ``VHOST_USER_SET_VRING_ENABLE``. -If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the -ring starts directly in the enabled state. - -If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is -initialized in a disabled state and is enabled by -``VHOST_USER_SET_VRING_ENABLE`` with parameter 1. +Between initialization and the first ``VHOST_USER_SET_FEATURES`` call, it +is implementation-defined whether each ring is enabled or disabled. + +If ``VHOST_USER_SET_FEATURES`` does not negotiate +``VHOST_USER_F_PROTOCOL_FEATURES``, each ring, when started, will be +enabled immediately. + +If ``VHOST_USER_SET_FEATURES`` does negotiate +``VHOST_USER_F_PROTOCOL_FEATURES``, each ring will remain in the disabled +state until ``VHOST_USER_SET_VRING_ENABLE`` enables it with parameter 1. + +Back-end implementations that support ``VHOST_USER_F_PROTOCOL_FEATURES`` +should implement this by initializing each ring in a disabled state, and +enabling them when ``VHOST_USER_SET_FEATURES`` is used without +negotiating ``VHOST_USER_F_PROTOCOL_FEATURES``. Other than that, rings +should only be enabled and disabled through +``VHOST_USER_SET_VRING_ENABLE``. While processing the rings (whether they are enabled or not), the back-end must support changing some configuration aspects on the fly.