Hi Alex, > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > Sent: Friday, June 21, 2019 11:58 PM > To: Liu, Yi L <yi.l.liu@xxxxxxxxx> > Subject: Re: [PATCH v1 9/9] smaples: add vfio-mdev-pci driver > > On Fri, 21 Jun 2019 10:23:10 +0000 > "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote: > > > Hi Alex, > > > > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > > > Sent: Friday, June 21, 2019 5:08 AM > > > To: Liu, Yi L <yi.l.liu@xxxxxxxxx> > > > Subject: Re: [PATCH v1 9/9] smaples: add vfio-mdev-pci driver > > > > > > On Thu, 20 Jun 2019 13:00:34 +0000 > > > "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote: > > > > > > > Hi Alex, > > > > > > > > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > > > > > Sent: Thursday, June 20, 2019 12:27 PM > > > > > To: Liu, Yi L <yi.l.liu@xxxxxxxxx> > > > > > Subject: Re: [PATCH v1 9/9] smaples: add vfio-mdev-pci driver > > > > > > > > > > On Sat, 8 Jun 2019 21:21:11 +0800 > > > > > Liu Yi L <yi.l.liu@xxxxxxxxx> wrote: > > > > > > > > > > > This patch adds sample driver named vfio-mdev-pci. It is to wrap > > > > > > a PCI device as a mediated device. For a pci device, once bound > > > > > > to vfio-mdev-pci driver, user space access of this device will > > > > > > go through vfio mdev framework. The usage of the device follows > > > > > > mdev management method. e.g. user should create a mdev before > > > > > > exposing the device to user-space. > > [...] > > > > > > > > > However, the patch below just makes the mdev interface behave > > > > > correctly, I can't make it work on my system because commit > > > > > 7bd50f0cd2fd ("vfio/type1: Add domain at(de)taching group helpers") > > > > > > > > What error did you encounter. I tested the patch with a device in a > > > > singleton iommu group. I'm also searching a proper machine with > > > > multiple devices in an iommu group and test it. > > > > > > In vfio_iommu_type1, iommu backed mdev devices use the > > > iommu_attach_device() interface, which includes: > > > > > > if (iommu_group_device_count(group) != 1) > > > goto out_unlock; > > > > > > So it's impossible to use with non-singleton groups currently. > > > > Hmmm, I think it is no longer good to use iommu_attach_device() for iommu > > backed mdev devices now. In this flow, the purpose here is to attach a device > > to a domain and no need to check whether the device is in a singleton iommu > > group. I think it would be better to use __iommu_attach_device() instead of > > iommu_attach_device(). > > That's a static and unexported, it's intentionally not an exposed > interface. We can't attach devices in the same group to separate > domains allocated through iommu_domain_alloc(), this would violate the > iommu group isolation principles. Go it. :-) Then not good to expose such interface. But to support devices in non-singleton iommu group, we need to have a new interface which doesn't count the devices but attach all the devices. > > Also I found a potential mutex lock issue if using iommu_attach_device(). > > In vfio_iommu_attach_group(), it uses iommu_group_for_each_dev() to loop > > all the devices in the group. It holds group->mutex. And then > vfio_mdev_attach_domain() > > calls iommu_attach_device() which also tries to get group->mutex. This would be > > an issue. If you are fine with it, I may post another patch for it. :-) > > Gack, yes, please send a patch. Would do it, may be together with the support of vfio-mdev-pci on devices in non-singleton iommu group. > > > > > > used iommu_attach_device() rather than iommu_attach_group() for non-aux > > > > > mdev iommu_device. Is there a requirement that the mdev parent device > > > > > is in a singleton iommu group? > > > > > > > > I don't think there should have such limitation. Per my understanding, > > > > vfio-mdev-pci should also be able to bind to devices which shares > > > > iommu group with other devices. vfio-pci works well for such devices. > > > > And since the two drivers share most of the codes, I think vfio-mdev-pci > > > > should naturally support it as well. > > > > > > Yes, the difference though is that vfio.c knows when devices are in the > > > same group, which mdev vfio.c only knows about the non-iommu backed > > > group, not the group that is actually used for the iommu backing. So > > > we either need to enlighten vfio.c or further abstract those details in > > > vfio_iommu_type1.c. > > > > Not sure if it is necessary to introduce more changes to vfio.c or > > vfio_iommu_type1.c. If it's only for the scenario which two devices share an > > iommu_group, I guess it could be supported by using __iommu_attach_device() > > which has no device counting for the group. But maybe I missed something > > here. It would be great if you can elaborate a bit for it. :-) > > We need to use the group semantics, there's a reason > __iommu_attach_device() is not exposed, it's an internal helper. I > think there's no way around that we need to somewhere track the actual > group we're attaching to and have the smarts to re-use it for other > devices in the same group. Hmmm, exposing __iommu_attach_device() is not good, let's forget it. :-) > > > > > If this is a simplification, then > > > > > vfio-mdev-pci should not bind to devices where this is violated since > > > > > there's no way to use the device. Can we support it though? > > > > > > > > yeah, I think we need to support it. > > > > > > > > > If I have two devices in the same group and bind them both to > > > > > vfio-mdev-pci, I end up with three groups, one for each mdev device and > > > > > the original physical device group. vfio.c works with the mdev groups > > > > > and will try to match both groups to the container. vfio_iommu_type1.c > > > > > also works with the mdev groups, except for the point where we actually > > > > > try to attach a group to a domain, which is the only window where we use > > > > > the iommu_device rather than the provided group, but we don't record > > > > > that anywhere. Should struct vfio_group have a pointer to a reference > > > > > counted object that tracks the actual iommu_group attached, such that > > > > > we can determine that the group is already attached to the domain and > > > > > not try to attach again? > > > > > > > > Agreed, we need to avoid such duplicated attach. Instead of adding > > > > reference counted object in vfio_group. I'm also considering the logic > > > > below: > > > > Re-walked the code, I find the duplicated attach will happen on the vfio-mdev-pci > > device as vfio_mdev_attach_domain() only attaches the parent devices of > > iommu backed mdevs instead of all the devices within the physical iommu_group. > > While for a vfio-pci device, it will use iommu_attach_group() which attaches all the > > devices within the iommu backed group. The same with detach, > > vfio_mdev_detach_domain() detaches selective devices instead of all devices > within > > the iommu backed group. > > Yep, that's not good, for the non-aux case we need to follow the usual > group semantics or else we're limited to singleton groups. yep. > > > > > /* > > > > * Do this check in vfio_iommu_type1_attach_group(), after mdev_group > > > > * is initialized. > > > > */ > > > > if (vfio_group->mdev_group) { > > > > /* > > > > * vfio_group->mdev_group is true means vfio_group->iommu_group > > > > * is not the actual iommu_group which is going to be attached to > > > > * domain. To avoid duplicate iommu_group attach, needs to check if > > > > * the actual iommu_group. vfio_get_parent_iommu_group() is a > > > > * newly added helper function which returns the actual attach > > > > * iommu_group going to be attached for this mdev group. > > > > */ > > > > p_iommu_group = vfio_get_parent_iommu_group( > > > > vfio_group->iommu_group); > > > > list_for_each_entry(d, &iommu->domain_list, next) { > > > > if (find_iommu_group(d, p_iommu_group)) { > > > > mutex_unlock(&iommu->lock); > > > > // skip group attach; > > > > } > > > > } > > > > > > We don't currently create a struct vfio_group for the parent, only for > > > the mdev iommu group. The iommu_attach for an iommu backed mdev > > > doesn't leave any traces of where it is actually attached, we just > > > count on retracing our steps for the detach. That's why I'm thinking > > > we need an object somewhere to track it and it needs to be reference > > > counted so that if both a vfio-mdev-pci device and a vfio-pci device > > > are using it, we leave it in place if either one is removed. > > > > Hmmm, here we are talking about tracking in iommu_group level though > > no good idea on where the object should be placed yet. However, we may > > need to tack in device level as I mentioned in above paragraph. If not, > > there may be sequence issue. e.g. if vfio-mdev-pci device is attached > > firstly, then the object will be initialized, and when vfio-pci device is > > attached, we will find the attach should be skipped and just inc the ref count. > > But actually it should not be skipped since the vfio-mdev-pci attach does not > > attach all devices within the iommu backed group. > > We can't do that though, the entire group needs to be attached. Agree, may be getting another interface which is similar with iommu_attach_device(), but works for devices which is in non-singleton groups. So the attach for iommu backed mdev will also result in a sound attach to all the devices which share iommu group with the parent device. This is just like vfio-pci devices. For the object for tracking purpose may be as below: struct vfio_iommu_object { struct iommu_group *group; struct kref kref; }; And I think it should be per-domain and per-iommu backed group since aux-domain support allows a iommu backed group to be attached to multiple domains. I'm considering if it is ok to have a list in vfio_domain. Before each domain attach, vfio should do a check in the list if the iommu backed group has been attached already. For vfio-pci devices, use its iommu group to do a search in the list. For vfio-mdev-pci devices, use its parent devices iommu group to do a search. Thus avoid duplicate attach. Thoughts? > > What's more, regards to sIOV case, a parent devices may have multiple > > mdevs and the mdevs may be assigned to the same VM. Thus there will be multiple > > attach on this parent device. This also makes me believe track in device level would > > be better. > > The aux domain support essentially specifies that the device can be > attached to multiple domains, so I think we're ok for device-level > group attach there, but not for bare iommu backed devices. Thanks, Got it. Thanks, Yi Liu
