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 X-Spam-Level: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0BF89C433E6 for ; Tue, 23 Feb 2021 09:49:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D113D64E31 for ; Tue, 23 Feb 2021 09:49:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231742AbhBWJtv (ORCPT ); Tue, 23 Feb 2021 04:49:51 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:34219 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231675AbhBWJsA (ORCPT ); Tue, 23 Feb 2021 04:48:00 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614073593; 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: in-reply-to:in-reply-to:references:references; bh=8ua870MbcBKf1L7a60gclNoUI6ARWuqH3V7P7cCW35M=; b=ZZFEVrQI7E9Q7u+pcLXKP1scO2KD1K/eUSG5wkRYETPIfirMYYabtQWhskWZDpnjDzIpXP FJKF2yZM4y2tGxyayD97nQke7sH4DJO0/ZPQs2wfYhNVX5Bxmtnd8G5PieNKTy+/+SrUG4 eAbrWI3W03r6I5/hb2uO5cWqjdrxZ5w= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-350-VR4YQnlkPE2T6Czu-kVUpg-1; Tue, 23 Feb 2021 04:46:29 -0500 X-MC-Unique: VR4YQnlkPE2T6Czu-kVUpg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5646A100B3B4; Tue, 23 Feb 2021 09:46:28 +0000 (UTC) Received: from [10.72.13.6] (ovpn-13-6.pek2.redhat.com [10.72.13.6]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9B49619C45; Tue, 23 Feb 2021 09:46:22 +0000 (UTC) Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero To: "Michael S. Tsirkin" , Si-Wei Liu Cc: elic@nvidia.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org References: <1613735698-3328-1-git-send-email-si-wei.liu@oracle.com> <605e7d2d-4f27-9688-17a8-d57191752ee7@redhat.com> <20210223041740-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com> Date: Tue, 23 Feb 2021 17:46:20 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210223041740-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/2/23 下午5:25, Michael S. Tsirkin wrote: > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote: >> >> On 2/21/2021 8:14 PM, Jason Wang wrote: >>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote: >>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked >>>> for legacy") made an exception for legacy guests to reset >>>> features to 0, when config space is accessed before features >>>> are set. We should relieve the verify_min_features() check >>>> and allow features reset to 0 for this case. >>>> >>>> It's worth noting that not just legacy guests could access >>>> config space before features are set. For instance, when >>>> feature VIRTIO_NET_F_MTU is advertised some modern driver >>>> will try to access and validate the MTU present in the config >>>> space before virtio features are set. >>> >>> This looks like a spec violation: >>> >>> " >>> >>> The following driver-read-only field, mtu only exists if >>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the >>> driver to use. >>> " >>> >>> Do we really want to workaround this? >> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest? >> >> I think the point is, since there's legacy guest we'd have to support, this >> host side workaround is unavoidable. Although I agree the violating driver >> should be fixed (yes, it's in today's upstream kernel which exists for a >> while now). > Oh you are right: > > > static int virtnet_validate(struct virtio_device *vdev) > { > if (!vdev->config->get) { > dev_err(&vdev->dev, "%s failure: config access disabled\n", > __func__); > return -EINVAL; > } > > if (!virtnet_validate_features(vdev)) > return -EINVAL; > > if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) { > int mtu = virtio_cread16(vdev, > offsetof(struct virtio_net_config, > mtu)); > if (mtu < MIN_MTU) > __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU); I wonder why not simply fail here? > } > > return 0; > } > > And the spec says: > > > The driver MUST follow this sequence to initialize a device: > 1. Reset the device. > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device. > 3. Set the DRIVER status bit: the guest OS knows how to drive the device. > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration > fields to check that it can support the device before accepting it. > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step. > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not > support our subset of features and the device is unusable. > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup, > reading and possibly writing the device’s virtio configuration space, and population of virtqueues. > 8. Set the DRIVER_OK status bit. At this point the device is “live”. > > > Item 4 on the list explicitly allows reading config space before > FEATURES_OK. > > I conclude that VIRTIO_NET_F_MTU is set means "set in device features". So this probably need some clarification. "is set" is used many times in the spec that has different implications. Thanks > > Generally it is worth going over feature dependent config fields > and checking whether they should be present when device feature is set > or when feature bit has been negotiated, and making this clear. >