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Tsirkin" Subject: Re: [PATCH v2 1/1] virtio: write back F_VERSION_1 before validate Message-ID: <20211008155156.626e78b5.pasic@linux.ibm.com> In-Reply-To: <20211008085839-mutt-send-email-mst@kernel.org> References: <20211008123422.1415577-1-pasic@linux.ibm.com> <20211008085839-mutt-send-email-mst@kernel.org> Organization: IBM X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: AwLL2TwiE5bb7sR5YjTQOfnp3HJWl-Nz X-Proofpoint-ORIG-GUID: pXbbnkG0k_9qzQg33cFNFi_GxeNjjWsS X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475 definitions=2021-10-08_03,2021-10-07_02,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxlogscore=999 clxscore=1015 malwarescore=0 spamscore=0 priorityscore=1501 mlxscore=0 adultscore=0 bulkscore=0 impostorscore=0 lowpriorityscore=0 phishscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109230001 definitions=main-2110080081 Received-SPF: pass client-ip=148.163.158.5; envelope-from=pasic@linux.ibm.com; helo=mx0b-001b2d01.pphosted.com X-Spam_score_int: -19 X-Spam_score: -2.0 X-Spam_bar: -- X-Spam_report: (-2.0 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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, Christian Borntraeger , qemu-devel@nongnu.org, Jason Wang , Cornelia Huck , linux-kernel@vger.kernel.org, stable@vger.kernel.org, virtualization@lists.linux-foundation.org, Halil Pasic , Xie Yongji , stefanha@redhat.com, Raphael Norwitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, 8 Oct 2021 09:05:03 -0400 "Michael S. Tsirkin" wrote: > On Fri, Oct 08, 2021 at 02:34:22PM +0200, 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: "... the driver MAY read (but MUST > > NOT write) the device-specific configuration fields to check that it can > > support the device ..." before setting FEATURES_OK. > > > > 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 a regression so let's try to address > > actually, regressions. and we can add > "since originally before validate callback existed > config space was only read after > FEATURES_OK. See Fixes tags for relevant commits" > > > it. How about replacing the paragraph above with the following? "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. Before validate callback existed, config space was only read after FEATURES_OK. However, we already have two regression, so let's address this here as well." > > > > The regressions affect the VIRTIO_NET_F_MTU feature of virtio-net and > > the VIRTIO_BLK_F_BLK_SIZE feature of virtio-blk for BE guests when > > virtio 1.0 is used on both sides. The latter renders virtio-blk > > unusable with DASD backing, because things simply don't work with > > the default. and add "See Fixes tags for relevant commits." here. > > Let's add a work around description now: > > > For QEMU, we can work around the issue by writing out the features > register with VIRTIO_F_VERSION_1 bit set. We (ab) use the s/features register/feature bits/ rationale: ccw does not have a features register, and qemu does not really act as if its behavior was controlled by the values in a features register. I.e. when we read the register we see VIRTIO_F_VERSION_! because the feature is offered. In QEMU we basically read host_featues but write the guest_features. And what drives device behavior is mostly guest_features. s/(ab) use/(ab)use/ > finalize_features config op for this. It's not enough to address vhost s/It's/This is/ > user and vhost block devices since these do not get the features until s/vhost user and vhost block/some vhost-user and vhost-vdpa/ ? Ratioale: I think vhost block is just a vhost-user device. On the other hand vhost-user-fs works like charm because the config space is implemented in qemu and not in the vhost-user device. I didn't check vhost_net. I'm not even sure qemu offers a vhost_net implementation. Anyway I wouldn't like to make any false statements here. > FEATURES_OK, however it looks like these two actually never handled the > endian-ness for legacy mode correctly, so at least that's not a > regression. > > No devices except virtio net and virtio blk seem to be affected. > > Long term the right thing to do is to fix the hypervisors. > Sounds good. Thanks! Are you OK with my changes proposed to your changes? Regards, Halil > > > > > Cc: #v4.11 > > 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 | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c > > index 0a5b54034d4b..236081afe9a2 100644 > > --- a/drivers/virtio/virtio.c > > +++ b/drivers/virtio/virtio.c > > @@ -239,6 +239,17 @@ 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 config space accesses before > > FEATURES_OK for > > + * these when needed. > > + */ > > + if (drv->validate && !virtio_legacy_is_little_endian() > > + && device_features & > > BIT_ULL(VIRTIO_F_VERSION_1)) { > > + 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 > > > > base-commit: 60a9483534ed0d99090a2ee1d4bb0b8179195f51 > > -- > > 2.25.1 >