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Tsirkin" , Jason Wang , Xie Yongji , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] virtio: write back F_VERSION_1 before validate In-Reply-To: <20211006142533.2735019-1-pasic@linux.ibm.com> Organization: Red Hat GmbH References: <20211006142533.2735019-1-pasic@linux.ibm.com> User-Agent: Notmuch/0.32.1 (https://notmuchmail.org) Date: Thu, 07 Oct 2021 13:52:24 +0200 Message-ID: <875yu9yruv.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Received-SPF: pass client-ip=170.10.133.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.05, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-s390@vger.kernel.org, markver@us.ibm.com, qemu-devel@nongnu.org, Raphael Norwitz , Halil Pasic , Christian Borntraeger , stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Oct 06 2021, Halil Pasic wrote: > The virtio specification virtio-v1.1-cs01 states: Transitional devices > MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not > been acknowledged by the driver. This is exactly what QEMU as of 6.1 > has done relying solely on VIRTIO_F_VERSION_1 for detecting that. > > However, the specification also says: driver MAY read (but MUST NOT > write) the device-specific configuration fields to check that it can > support the device before setting FEATURES_OK. Suggest to put the citations from the spec into quotes, so that they are distinguishable from the rest of the text. > > In that case, any transitional device relying solely on > VIRTIO_F_VERSION_1 for detecting legacy drivers will return data in > legacy format. In particular, this implies that it is in big endian > format for big endian guests. This naturally confuses the driver which > expects little endian in the modern mode. > > It is probably a good idea to amend the spec to clarify that > VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation > is complete. However, we already have regression so let's try to address s/regression/a regression/ > it. Maybe mention what the regression is? Also mention that we use this workaround for modern on BE only? > > Signed-off-by: Halil Pasic > Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space") > Fixes: fe36cbe0671e ("virtio_net: clear MTU when out of range") > Reported-by: markver@us.ibm.com > --- > drivers/virtio/virtio.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c > index 0a5b54034d4b..494cfecd3376 100644 > --- a/drivers/virtio/virtio.c > +++ b/drivers/virtio/virtio.c > @@ -239,6 +239,16 @@ static int virtio_dev_probe(struct device *_d) > driver_features_legacy = driver_features; > } > > + /* > + * Some devices detect legacy solely via F_VERSION_1. Write > + * F_VERSION_1 to force LE for these when needed. "...to force LE config space accesses before FEATURES_OK for these when needed (BE)." ? > + */ > + if (drv->validate && !virtio_legacy_is_little_endian() > + && BIT_ULL(VIRTIO_F_VERSION_1) & device_features) { Nit: putting device_features first would read more naturally to me. > + dev->features = BIT_ULL(VIRTIO_F_VERSION_1); > + dev->config->finalize_features(dev); > + } > + > if (device_features & (1ULL << VIRTIO_F_VERSION_1)) > dev->features = driver_features & device_features; > else Patch LGTM.