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[189.68.153.170]) by smtp.gmail.com with ESMTPSA id l15sm7010556ots.28.2022.02.02.08.27.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Feb 2022 08:27:46 -0800 (PST) Message-ID: <1bba9ca1-99c9-96d3-f226-b7b9e2ff6f55@gmail.com> Date: Wed, 2 Feb 2022 13:27:40 -0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported Content-Language: en-US To: Halil Pasic 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> From: Daniel Henrique Barboza In-Reply-To: <20220202172353.65ad8325.pasic@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Host-Lookup-Failed: Reverse DNS lookup failed for 2607:f8b0:4864:20::334 (failed) Received-SPF: pass client-ip=2607:f8b0:4864:20::334; envelope-from=danielhb413@gmail.com; helo=mail-ot1-x334.google.com X-Spam_score_int: -10 X-Spam_score: -1.1 X-Spam_bar: - X-Spam_report: (-1.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, NICE_REPLY_A=-0.001, PDS_HP_HELO_NORDNS=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RDNS_NONE=0.793, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no 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 , Brijesh Singh , "Michael S. Tsirkin" , Jason Wang , Cornelia Huck , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Jakob Naucke Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2/2/22 13:23, 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. Agree. I'll re-ack the patch given that I did it from another email that isn't in QEMU devel. All this discussion is pertinent to a separated work. Thanks, Daniel > > 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