linux-kernel - Re: [PATCH v2 1/2] iommu: iommu_aux_at(de)tach

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Message-ID: <20200708130749.1b1e1421@x1.home>
Date:   Wed, 8 Jul 2020 13:07:49 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Cornelia Huck <cohuck@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iommu: iommu_aux_at(de)tach_device() extension

On Wed, 8 Jul 2020 10:53:12 +0800
Lu Baolu <baolu.lu@...ux.intel.com> wrote:

> Hi Alex,
> 
> Thanks a lot for your comments. Please check my reply inline.
> 
> On 7/8/20 5:04 AM, Alex Williamson wrote:
> > On Tue,  7 Jul 2020 09:39:56 +0800
> > Lu Baolu<baolu.lu@...ux.intel.com>  wrote:
> >   
> >> The hardware assistant vfio mediated device is a use case of iommu
> >> aux-domain. The interactions between vfio/mdev and iommu during mdev
> >> creation and passthr are:
> >>
> >> - Create a group for mdev with iommu_group_alloc();
> >> - Add the device to the group with
> >>          group = iommu_group_alloc();
> >>          if (IS_ERR(group))
> >>                  return PTR_ERR(group);
> >>
> >>          ret = iommu_group_add_device(group, &mdev->dev);
> >>          if (!ret)
> >>                  dev_info(&mdev->dev, "MDEV: group_id = %d\n",
> >>                           iommu_group_id(group));
> >> - Allocate an aux-domain
> >>          iommu_domain_alloc()
> >> - Attach the aux-domain to the physical device from which the mdev is
> >>    created.
> >>          iommu_aux_attach_device()
> >>
> >> In the whole process, an iommu group was allocated for the mdev and an
> >> iommu domain was attached to the group, but the group->domain leaves
> >> NULL. As the result, iommu_get_domain_for_dev() doesn't work anymore.
> >>
> >> The iommu_get_domain_for_dev() is a necessary interface for device
> >> drivers that want to support aux-domain. For example,
> >>
> >>          struct iommu_domain *domain;
> >>          struct device *dev = mdev_dev(mdev);
> >>          unsigned long pasid;
> >>
> >>          domain = iommu_get_domain_for_dev(dev);
> >>          if (!domain)
> >>                  return -ENODEV;
> >>
> >>          pasid = iommu_aux_get_pasid(domain, dev->parent);  
> > How did we know this was an aux domain? ie. How did we know we could
> > use it with iommu_aux_get_pasid()?  
> 
> Yes. It's a bit confusing if iommu_get_domain_for_dev() is reused here
> for aux-domain.
> 
> > 
> > Why did we assume the parent device is the iommu device for the aux
> > domain?  Should that level of detail be already known by the aux domain?
> > 
> > Nits - The iomu device of an mdev device is found via
> > mdev_get_iommu_device(dev), it should not be assumed to be the parent.
> > The parent of an mdev device is found via mdev_parent_dev(mdev).  
> 
> My bad. The driver should use mdev_get_iommu_device() instead.
> 
> > 
> > The leaps in logic here make me wonder if we should instead be exposing
> > more of an aux domain API rather than blurring the differences between
> > these domains.  Thanks,  
> 
> How about add below API?
> 
> /**
>   * iommu_aux_get_domain_for_dev - get aux domain for a device
>   * @dev: the accessory device
>   *
>   * The caller should pass a valid @dev to iommu_aux_attach_device() before
>   * calling this api. Return an attached aux-domain, or NULL otherwise.

That's not necessarily the caller's responsibility, that might happen
elsewhere, this function simply returns an aux domain for the device if
it's attached to one.

>   */
> struct iommu_domain *iommu_aux_get_domain_for_dev(struct device *dev)
> {
>          struct iommu_domain *domain = NULL;
>          struct iommu_group *group;
> 
>          group = iommu_group_get(dev);
>          if (!group)
>                  return NULL;
> 
>          if (group->aux_domain_attached)
>                  domain = group->domain;
> 
>          iommu_group_put(group);
> 
>          return domain;
> }
> EXPORT_SYMBOL_GPL(iommu_aux_get_domain_for_dev);

For your example use case, this seems more clear to me.  Thanks,

Alex