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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 5789BC433E0 for ; Tue, 23 Feb 2021 10:09:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1BB1E64E3F for ; Tue, 23 Feb 2021 10:09:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232430AbhBWKI4 (ORCPT ); Tue, 23 Feb 2021 05:08:56 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:46073 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232389AbhBWKH5 (ORCPT ); Tue, 23 Feb 2021 05:07:57 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614074790; 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=U9ezWLhwEDgAZeIF3d1ynnogTVpyyQruaUJOC2rtSJM=; b=DAcYZZjdsgfxl8fMtUYRLBXi8oGnanM5o9p7YuTe+l5/IK6f/hrL/rS+dUMijY59yafRTa /9KEEy7nlzoB13gWzATg+gC+MgN9ZpNJXh/hBWkdlDIr8HGPO7/LO2rl+4i9XcOHMYE75I eA4mjrxvo+yiWBvw1R9cSrhQGQE4TQY= 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-408-7btmxPb9NoKnV01bpqZfTg-1; Tue, 23 Feb 2021 05:04:39 -0500 X-MC-Unique: 7btmxPb9NoKnV01bpqZfTg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4025780403E; Tue, 23 Feb 2021 10:04:38 +0000 (UTC) Received: from gondolin (ovpn-113-126.ams2.redhat.com [10.36.113.126]) by smtp.corp.redhat.com (Postfix) with ESMTP id 828665F706; Tue, 23 Feb 2021 10:04:33 +0000 (UTC) Date: Tue, 23 Feb 2021 11:04:30 +0100 From: Cornelia Huck To: Jason Wang Cc: "Michael S. Tsirkin" , Si-Wei Liu , elic@nvidia.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero Message-ID: <20210223110430.2f098bc0.cohuck@redhat.com> In-Reply-To: <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com> 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> <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, 23 Feb 2021 17:46:20 +0800 Jason Wang wrote: > On 2021/2/23 =E4=B8=8B=E5=8D=885:25, Michael S. Tsirkin wrote: > > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote: =20 > >> > >> On 2/21/2021 8:14 PM, Jason Wang wrote: =20 > >>> On 2021/2/19 7:54 =E4=B8=8B=E5=8D=88, Si-Wei Liu wrote: =20 > >>>> 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. =20 > >>> > >>> 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? =20 > >> 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 dr= iver > >> should be fixed (yes, it's in today's upstream kernel which exists for= a > >> while now). =20 > > Oh you are right: > > > > > > static int virtnet_validate(struct virtio_device *vdev) > > { > > if (!vdev->config->get) { > > dev_err(&vdev->dev, "%s failure: config access disable= d\n", > > __func__); > > return -EINVAL; > > } > > > > if (!virtnet_validate_features(vdev)) > > return -EINVAL; > > > > if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) { > > int mtu =3D virtio_cread16(vdev, > > offsetof(struct virtio_net_co= nfig, > > mtu)); > > if (mtu < MIN_MTU) > > __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU); =20 >=20 >=20 > I wonder why not simply fail here? I think both failing or not accepting the feature can be argued to make sense: "the device presented us with a mtu size that does not make sense" would point to failing, "we cannot work with the mtu size that the device presented us" would point to not negotiating the feature. >=20 >=20 > > } > > > > 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 devic= e. > > 4. Read device feature bits, and write the subset of feature bits under= stood by the OS and driver to the > > device. During this step the driver MAY read (but MUST NOT write) the d= evice-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 featu= re bits after this step. > > 6. Re-read device status to ensure the FEATURES_OK bit is still set: ot= herwise, 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=E2=80=99s virtio configuration = space, and population of virtqueues. > > 8. Set the DRIVER_OK status bit. At this point the device is =E2=80=9Cl= ive=E2=80=9D. > > > > > > 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".= =20 >=20 >=20 > So this probably need some clarification. "is set" is used many times in= =20 > the spec that has different implications. Before FEATURES_OK is set by the driver, I guess it means "the device has offered the feature"; during normal usage, it means "the feature has been negotiated". (This is a bit fuzzy for legacy mode.) Should we add a wording clarification to the spec?