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Tsirkin" To: Halil Pasic Subject: Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported Message-ID: <20220202115034-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> <20220202172353.65ad8325.pasic@linux.ibm.com> MIME-Version: 1.0 In-Reply-To: <20220202172353.65ad8325.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-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: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.086, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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, Jakob Naucke , qemu-stable@nongnu.org, Daniel Henrique Barboza Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Feb 02, 2022 at 05:23:53PM +0100, Halil Pasic wrote: > On Wed, 2 Feb 2022 10:24:51 -0300 > Daniel Henrique Barboza wrote: > > > On 2/1/22 22:15, Halil Pasic wrote: > > > On Tue, 1 Feb 2022 16:31:22 -0300 > > > Daniel Henrique Barboza wrote: > > > > > >> On 2/1/22 15:33, Halil Pasic wrote: > > >>> On Tue, 1 Feb 2022 12:36:25 -0300 > > >>> Daniel Henrique Barboza wrote: > > >>> > > >>>>> + 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, > > >>>>> + "iommu_platform=true is not supported by the device"); > > >>>>> + } > > >>>> > > >>>> > > >>>>> } else { > > >>>>> vdev->dma_as = &address_space_memory; > > >>>>> } > > >>>> > > >>>> > > >>>> I struggled to understand what this 'else' clause was doing and I assumed that it was > > >>>> wrong. Searching through the ML I learned that this 'else' clause is intended to handle > > >>>> legacy virtio devices that doesn't support the DMA API (introduced in 8607f5c3072caeebb) > > >>>> and thus shouldn't set VIRTIO_F_IOMMU_PLATFORM. > > >>>> > > >>>> > > >>>> My suggestion, if a v4 is required for any other reason, is to add a small comment in this > > >>>> 'else' clause explaining that this is the legacy virtio devices condition and those devices > > >>>> don't set F_IOMMU_PLATFORM. This would make the code easier to read for a virtio casual like > > >>>> myself. > > >>> > > >>> I do not agree that this is about legacy virtio. In my understanding > > >>> virtio-ccw simply does not need translation because CCW devices use > > >>> guest physical addresses as per architecture. It may be considered > > >>> legacy stuff form PCI perspective, but I don't think it is legacy > > >>> in general. > > >> > > >> > > >> I wasn't talking about virtio-ccw. I was talking about this piece of code: > > >> > > >> > > >> 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); > > >> } else { > > >> vdev->dma_as = &address_space_memory; > > >> } > > >> > > >> > > >> I suggested something like this: > > >> > > >> > > >> > > >> 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); > > >> } else { > > >> /* > > >> * We don't force VIRTIO_F_IOMMU_PLATFORM for legacy devices, i.e. > > >> * devices that don't implement klass->get_dma_as, regardless of > > >> * 'has_iommu' setting. > > >> */ > > >> vdev->dma_as = &address_space_memory; > > >> } > > >> > > >> > > >> At least from my reading of commits 8607f5c3072 and 2943b53f682 this seems to be > > >> the case. I spent some time thinking that this IF/ELSE was wrong because I wasn't > > >> aware of this history. > > > > > > With virtio-ccw we take the else branch because we don't implement > > > ->get_dma_as(). I don't consider all the virtio-ccw to be legacy. > > > > > > IMHO there are two ways to think about this: > > > a) The commit that introduced this needs a fix which implemets > > > get_dma_as() for virtio-ccw in a way that it simply returns > > > address_space_memory. > > > b) The presence of ->get_dma_as() is not indicative of "legacy". > > > > > > BTW in virtospeak "legacy" has a special meaning: pre-1.0 virtio. Do you > > > mean that legacy. And if I read the virtio-pci code correctly > > > ->get_dma_as is set for legacy, transitional and modern devices alike. > > > > > > Oh ok. I'm not well versed into virtiospeak. My "legacy" comment was a poor choice of > > word for the situation. > > > > We can ignore the "legacy" bit. My idea/suggestion is to put a comment at that point > > explaining the logic behind into not forcing VIRTIO_F_IOMMU_PLATFORM in devices that > > doesn't implement ->get_dma_as(). > > > > I am assuming that this is an intended design that was introduced by 2943b53f682 > > ("virtio: force VIRTIO_F_IOMMU_PLATFORM"), meaning that the implementation of the > > ->get_dma_as is being used as a parameter to force the feature in the device. And with > > this code: > > > > > > 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); > > } else { > > vdev->dma_as = &address_space_memory; > > } > > > > It is possible that we have 2 vdev devices where ->dma_as = &address_space_memory, but one > > of them is sitting in a bus where "klass->get_dma_as(qbus->parent) = &address_space_memory", > > and this device will have VIRTIO_F_IOMMU_PLATFORM forced onto it and the former won't. > > > > > > If this is not an intended design I can only speculate how to fix it. Forcing VIRTIO_F_IOMMU_PLATFORM > > in all devices, based only on has_iommu, can break stuff. Setting VIRTIO_F_IOMMU_PLATFORM only > > if "vdev->dma_as != &address_space_memory" make some sense but I am fairly certain it will > > break stuff the other way. Or perhaps the fix is something else entirely. > > > > > > > > > > > > > > IMHO the important thing to figure out is what impact that > > > virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > > in the first branch (of the if-else) has. IMHO if one examines the > > > commits 8607f5c307 ("virtio: convert to use DMA api") and 2943b53f68 > > > ("virtio: force VIRTIO_F_IOMMU_PLATFORM") very carefully, one will > > > probably reach the conclusion that the objective of the latter, is > > > to prevent the guest form not negotiating the IOMMU_PLATFORM feature > > > (clearing it as part of the feature negotiation) and trying to use > > > the device without that feature. In other words, virtio features are > > > usually optional for the guest for the sake of compatibility, but > > > IOMMU_PLATFORM is not: for very good reasons. Neither the commit message > > > nor the patch does mention legacy anywhere. > > > > > > 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. > > > > Yeah, this go back on what I've said about 2943b53f682 up there. There are assumptions > > being made on the ->get_dma_as() existence that aren't clear. > > > > I agree. The commit message does not explain. > > > > > > > > > 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); > > > } > > > > > > I am fairly confident that forcing VIRTIO_F_IOMMU_PLATFORM all around, based on has_iommu > > Yes I should have made that conditional on has_iommu. It was very late > when I finished that email. > > > alone, will have consequences all around. This code has been around for almost 5 years and a > > lot of stuff has been developed on top of it. > > > > Do you have any particular examples in mind? > > > All that said, if this is the proper way of fixing it I'd say to do it now, document it properly > > and fix the breakages as they come along. The alternative - hacking around and around a codebase > > that might not be solid - is worse in the long run. > > IMHO this is a good discussion you triggered. But I see it out of scope > for the bug I'm trying to fix. > > I can post a proper patch for "IOMMU_PLATFORM is non-negotiable for > all guests" and we can have proper review and discussion on that. But > I would like the bug I'm working on here fixed first. There are > people that want to use virtiofs with confidential guests, and > we should really make sure they can. > > Regards, > Halil I think I second that. -- MST