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Tsirkin" To: Christian Borntraeger Subject: Re: [PATCH v3 1/1] virtio: write back F_VERSION_1 before validate Message-ID: <20211013081836-mutt-send-email-mst@kernel.org> References: <20211011053921.1198936-1-pasic@linux.ibm.com> <20211013060923-mutt-send-email-mst@kernel.org> <96561e29-e0d6-9a4d-3657-999bad59914e@de.ibm.com> MIME-Version: 1.0 In-Reply-To: <96561e29-e0d6-9a4d-3657-999bad59914e@de.ibm.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -8 X-Spam_score: -0.9 X-Spam_bar: / X-Spam_report: (-0.9 / 5.0 requ) DKIMWL_WL_HIGH=-0.049, 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, 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 Wed, Oct 13, 2021 at 01:23:50PM +0200, Christian Borntraeger wrote: > > > Am 13.10.21 um 12:10 schrieb Michael S. Tsirkin: > > On Mon, Oct 11, 2021 at 07:39:21AM +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. Before validate callback existed, config space was only > > > read after FEATURES_OK. However, we already have two regressions, 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. > > > See Fixes tags for relevant commits. > > > > > > For QEMU, we can work around the issue by writing out the feature bits > > > with VIRTIO_F_VERSION_1 bit set. We (ab)use the finalize_features > > > config op for this. This isn't enough to address all vhost devices since > > > these do not get the features until FEATURES_OK, however it looks like > > > the affected devices actually never handled the endianness 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. > > > > > > 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 > > > Reviewed-by: Cornelia Huck > > > > OK this looks good! How about a QEMU patch to make it spec compliant on > > BE? > > Who is going to do that? Halil? you? Conny? Halil said he'll do it... Right, Halil? > Can we get this kernel patch queued for 5.15 and stable without waiting for the QEMU patch > as we have a regression with 4.14? Probably. Still trying to decide between this and plain revert for 5.15 and back. Maybe both? > > > > > --- > > > > > > @Connie: I made some more commit message changes to accommodate Michael's > > > requests. I just assumed these will work or you as well and kept your > > > r-b. Please shout at me if it needs to be dropped :) > > > --- > > > 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 > >