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Tsirkin" Subject: Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported Message-ID: <20220202141645.1f784f19.pasic@linux.ibm.com> In-Reply-To: <20220202020543-mutt-send-email-mst@kernel.org> References: <20220201133915.3764972-1-pasic@linux.ibm.com> <20220201193309.7da86258.pasic@linux.ibm.com> <365305e3-4224-965d-2cb6-496a95802f0e@gmail.com> <20220202021547.20dc65c9.pasic@linux.ibm.com> <20220202020543-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: QYIZisRXSguBcPXYO_yfN8FBT3DqzeZZ X-Proofpoint-ORIG-GUID: PYoc40tdDYHU93S4xAQaqoJpjvj0aNNv X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.816,Hydra:6.0.425,FMLib:17.11.62.513 definitions=2022-02-02_06,2022-02-01_01,2021-12-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 mlxlogscore=999 malwarescore=0 mlxscore=0 lowpriorityscore=0 impostorscore=0 spamscore=0 clxscore=1015 suspectscore=0 bulkscore=0 adultscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2201110000 definitions=main-2202020072 Received-SPF: pass client-ip=148.163.156.1; envelope-from=pasic@linux.ibm.com; helo=mx0a-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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Cornelia Huck , Brijesh Singh , Jason Wang , Daniel Henrique Barboza , qemu-devel@nongnu.org, qemu-stable@nongnu.org, Halil Pasic , Daniel Henrique Barboza , Jakob Naucke Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 2 Feb 2022 02:06:12 -0500 "Michael S. Tsirkin" wrote: [..] > > In my opinion not forcing the guest to negotiate IOMMU_PLATFORM when > > ->get_dma_as() is not set is at least unfortunate. Please observe, that > > virtio-pci is not affected by this omission because for virtio-pci > > devices ->get_dma_as != NULL always holds. And what is the deal for > > devices that don't implement get_dma_as() (and don't need address > > translation)? If iommu_platform=on is justified (no user error) then > > the device does not have access to the entire guest memory. Which > > means it more than likely needs cooperation form the guest (driver). > > So detecting that the guest does not support IOMMU_PLATFORM and failing > > gracefully via virtio_validate_features() instead of carrying on > > in good faith and failing in ugly ways when the host attempts to access > > guest memory to which it does not have access to. If we assume user > > error, that is the host can access at least all the memory it needs > > to access to make that device work, then it is probably still a > > good idea to fail the device and thus help the user correct his > > error. > > > > IMHO the best course of action is > > diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c > > index 34f5a0a664..1d0eb16d1c 100644 > > --- a/hw/virtio/virtio-bus.c > > +++ b/hw/virtio/virtio-bus.c > > @@ -80,7 +80,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) > > > > vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); > > if (klass->get_dma_as != NULL && has_iommu) { > > - virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > vdev->dma_as = klass->get_dma_as(qbus->parent); > > if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) { > > error_setg(errp, > > @@ -89,6 +88,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) > > } else { > > vdev->dma_as = &address_space_memory; > > } > > + virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > } > > > > which would be a separate patch, as this is a separate issue. Jason, > > Michael, Connie, what do you think? > > Do you mean just force VIRTIO_F_IOMMU_PLATFORM for everyone? > Or am I misreading the patch? Yes. Where force means: prevent the driver from setting FEATURES_OK if it cleared VIRTIO_F_IOMMU_PLATFORM. I really don't see the case where the device offering but the driver not accepting VIRTIO_F_IOMMU_PLATFORM is good and useful. Regards, Halil > > > > Regards, > > Halil > >