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Tsirkin" To: Halil Pasic Subject: Re: [PATCH v3 1/1] virtio: write back F_VERSION_1 before validate Message-ID: <20211013060923-mutt-send-email-mst@kernel.org> References: <20211011053921.1198936-1-pasic@linux.ibm.com> MIME-Version: 1.0 In-Reply-To: <20211011053921.1198936-1-pasic@linux.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-Disposition: inline Cc: linux-s390@vger.kernel.org, markver@us.ibm.com, Christian Borntraeger , qemu-devel@nongnu.org, Cornelia Huck , linux-kernel@vger.kernel.org, stable@vger.kernel.org, virtualization@lists.linux-foundation.org, Xie Yongji , stefanha@redhat.com, Raphael Norwitz X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" 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? > --- > > @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 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization