Date:   Fri, 31 Jul 2020 12:05:37 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Cornelia Huck <cohuck@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v3 2/4] iommu: Add iommu_aux_at(de)tach_group()

On Fri, 31 Jul 2020 13:47:57 +0800
Lu Baolu <baolu.lu@...ux.intel.com> wrote:

> Hi Alex,
> 
> On 2020/7/31 3:46, Alex Williamson wrote:
> > On Wed, 29 Jul 2020 23:34:40 +0000
> > "Tian, Kevin" <kevin.tian@...el.com> wrote:
> >   
> >>> From: Alex Williamson <alex.williamson@...hat.com>
> >>> Sent: Thursday, July 30, 2020 4:04 AM
> >>>
> >>> On Thu, 16 Jul 2020 09:07:46 +0800
> >>> Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> >>>      
> >>>> Hi Jacob,
> >>>>
> >>>> On 7/16/20 12:01 AM, Jacob Pan wrote:  
> >>>>> On Wed, 15 Jul 2020 08:47:36 +0800
> >>>>> Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> >>>>>     
> >>>>>> Hi Jacob,
> >>>>>>
> >>>>>> On 7/15/20 12:39 AM, Jacob Pan wrote:  
> >>>>>>> On Tue, 14 Jul 2020 13:57:01 +0800
> >>>>>>> Lu Baolu<baolu.lu@...ux.intel.com>  wrote:
> >>>>>>>     
> >>>>>>>> This adds two new aux-domain APIs for a use case like vfio/mdev
> >>>>>>>> where sub-devices derived from an aux-domain capable device are
> >>>>>>>> created and put in an iommu_group.
> >>>>>>>>
> >>>>>>>> /**
> >>>>>>>>     * iommu_aux_attach_group - attach an aux-domain to an  
> >>> iommu_group  
> >>>>>>>> which
> >>>>>>>>     *                          contains sub-devices (for example
> >>>>>>>> mdevs) derived
> >>>>>>>>     *                          from @dev.
> >>>>>>>>     * @domain: an aux-domain;
> >>>>>>>>     * @group:  an iommu_group which contains sub-devices derived  
> >>> from  
> >>>>>>>> @dev;
> >>>>>>>>     * @dev:    the physical device which supports  
> >>> IOMMU_DEV_FEAT_AUX.  
> >>>>>>>>     *
> >>>>>>>>     * Returns 0 on success, or an error value.
> >>>>>>>>     */
> >>>>>>>> int iommu_aux_attach_group(struct iommu_domain *domain,
> >>>>>>>>                               struct iommu_group *group,
> >>>>>>>>                               struct device *dev)
> >>>>>>>>
> >>>>>>>> /**
> >>>>>>>>     * iommu_aux_detach_group - detach an aux-domain from an
> >>>>>>>> iommu_group *
> >>>>>>>>     * @domain: an aux-domain;
> >>>>>>>>     * @group:  an iommu_group which contains sub-devices derived  
> >>> from  
> >>>>>>>> @dev;
> >>>>>>>>     * @dev:    the physical device which supports  
> >>> IOMMU_DEV_FEAT_AUX.  
> >>>>>>>>     *
> >>>>>>>>     * @domain must have been attached to @group via
> >>>>>>>> iommu_aux_attach_group(). */
> >>>>>>>> void iommu_aux_detach_group(struct iommu_domain *domain,
> >>>>>>>>                                struct iommu_group *group,
> >>>>>>>>                                struct device *dev)
> >>>>>>>>
> >>>>>>>> It also adds a flag in the iommu_group data structure to identify
> >>>>>>>> an iommu_group with aux-domain attached from those normal ones.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> >>>>>>>> ---
> >>>>>>>>     drivers/iommu/iommu.c | 58
> >>>>>>>> +++++++++++++++++++++++++++++++++++++++++++  
> >>> include/linux/iommu.h |  
> >>>>>>>> 17 +++++++++++++ 2 files changed, 75 insertions(+)
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> >>>>>>>> index e1fdd3531d65..cad5a19ebf22 100644
> >>>>>>>> --- a/drivers/iommu/iommu.c
> >>>>>>>> +++ b/drivers/iommu/iommu.c
> >>>>>>>> @@ -45,6 +45,7 @@ struct iommu_group {
> >>>>>>>>     	struct iommu_domain *default_domain;
> >>>>>>>>     	struct iommu_domain *domain;
> >>>>>>>>     	struct list_head entry;
> >>>>>>>> +	unsigned int aux_domain_attached:1;
> >>>>>>>>     };
> >>>>>>>>
> >>>>>>>>     struct group_device {
> >>>>>>>> @@ -2759,6 +2760,63 @@ int iommu_aux_get_pasid(struct  
> >>> iommu_domain  
> >>>>>>>> *domain, struct device *dev) }
> >>>>>>>>     EXPORT_SYMBOL_GPL(iommu_aux_get_pasid);
> >>>>>>>>
> >>>>>>>> +/**
> >>>>>>>> + * iommu_aux_attach_group - attach an aux-domain to an  
> >>> iommu_group  
> >>>>>>>> which
> >>>>>>>> + *                          contains sub-devices (for example
> >>>>>>>> mdevs) derived
> >>>>>>>> + *                          from @dev.
> >>>>>>>> + * @domain: an aux-domain;
> >>>>>>>> + * @group:  an iommu_group which contains sub-devices derived  
> >>> from  
> >>>>>>>> @dev;
> >>>>>>>> + * @dev:    the physical device which supports  
> >>> IOMMU_DEV_FEAT_AUX.  
> >>>>>>>> + *
> >>>>>>>> + * Returns 0 on success, or an error value.
> >>>>>>>> + */
> >>>>>>>> +int iommu_aux_attach_group(struct iommu_domain *domain,
> >>>>>>>> +			   struct iommu_group *group, struct
> >>>>>>>> device *dev) +{
> >>>>>>>> +	int ret = -EBUSY;
> >>>>>>>> +
> >>>>>>>> +	mutex_lock(&group->mutex);
> >>>>>>>> +	if (group->domain)
> >>>>>>>> +		goto out_unlock;
> >>>>>>>> +  
> >>>>>>> Perhaps I missed something but are we assuming only one mdev per
> >>>>>>> mdev group? That seems to change the logic where vfio does:
> >>>>>>> iommu_group_for_each_dev()
> >>>>>>> 	iommu_aux_attach_device()
> >>>>>>>     
> >>>>>>
> >>>>>> It has been changed in PATCH 4/4:
> >>>>>>
> >>>>>> static int vfio_iommu_attach_group(struct vfio_domain *domain,
> >>>>>>                                       struct vfio_group *group)
> >>>>>> {
> >>>>>>            if (group->mdev_group)
> >>>>>>                    return iommu_aux_attach_group(domain->domain,
> >>>>>>                                                  group->iommu_group,
> >>>>>>                                                  group->iommu_device);
> >>>>>>            else
> >>>>>>                    return iommu_attach_group(domain->domain,
> >>>>>> group->iommu_group);
> >>>>>> }
> >>>>>>
> >>>>>> So, for both normal domain and aux-domain, we use the same concept:
> >>>>>> attach a domain to a group.
> >>>>>>     
> >>>>> I get that, but don't you have to attach all the devices within the  
> >>>>
> >>>> This iommu_group includes only mediated devices derived from an
> >>>> IOMMU_DEV_FEAT_AUX-capable device. Different from  
> >>> iommu_attach_group(),  
> >>>> iommu_aux_attach_group() doesn't need to attach the domain to each
> >>>> device in group, instead it only needs to attach the domain to the
> >>>> physical device where the mdev's were created from.
> >>>>     
> >>>>> group? Here you see the group already has a domain and exit.  
> >>>>
> >>>> If the (group->domain) has been set, that means a domain has already
> >>>> attached to the group, so it returns -EBUSY.  
> >>>
> >>> I agree with Jacob, singleton groups should not be built into the IOMMU
> >>> API, we're not building an interface just for mdevs or current
> >>> limitations of mdevs.  This also means that setting a flag on the group
> >>> and passing a device that's assumed to be common for all devices within
> >>> the group, don't really make sense here.  Thanks,
> >>>
> >>> Alex  
> >>
> >> Baolu and I discussed about this assumption before. The assumption is
> >> not based on singleton groups. We do consider multiple mdevs in one
> >> group. But our feeling at the moment is that all mdevs (or other AUX
> >> derivatives) in the same group should come from the same parent
> >> device, thus comes with above design. Does it sound a reasonable
> >> assumption to you?  
> > 
> > No, the approach in this series doesn't really make sense to me.  We
> > currently have the following workflow as Baolu notes in the cover
> > letter:
> > 
> > 	domain = iommu_domain_alloc(bus);
> > 
> > 	iommu_group_for_each_dev(group...
> > 
> > 		iommu_device = mdev-magic()
> > 
> > 		if (iommu_dev_feature_enabled(iommu_device,
> > 						IOMMU_DEV_FEAT_AUX))
> > 			iommu_aux_attach_device(domain, iommu_device);
> > 
> > And we want to convert this to a group function, like we have for
> > non-aux domains:
> > 
> > 	domain = iommu_domain_alloc(bus);
> > 
> > 	iommu_device = mdev-magic()
> > 
> > 	iommu_aux_attach_group(domain, group, iommu_device);
> > 
> > And I think we want to do that largely because iommu_group.domain is
> > private to iommu.c (therefore vfio code cannot set it), but we need it
> > set in order for iommu_get_domain_for_dev() to work with a group
> > attached to an aux domain.  Passing an iommu_device avoids the problem
> > that IOMMU API code doesn't know how to derive an iommu_device for each
> > device in the group, but while doing so it ignores the fundamental
> > nature of a group as being a set of one or more devices.  Even if we
> > can make the leap that all devices within the group would use the same
> > iommu_device, an API that sets and aux domain for a group while
> > entirely ignoring the devices within the group seems very broken.  
> 
> Agreed. We couldn't assume that all devices in an iommu group shares a
> same iommu_device, especially when it comes to PF/VF wrapped mediated
> device case.
> 
> > 
> > So, barring adding an abstraction at struct device where an IOMMU API
> > could retrieve the iommu_device backing anther device (which seems a
> > very abstract concept for the base class), why not have the caller
> > provide a lookup function?  Ex:
> > 
> > int iommu_aux_attach_group(struct iommu_domain *domain,
> > 			   struct iommu_group *group,
> > 			   struct device *(*iommu_device_lookup)(
> > 				struct device *dev));
> > 
> > Thus vfio could could simply provide &vfio_mdev_get_iommu_device and
> > we'd have equivalent functionality to what we have currently, but with
> > the domain pointer set in the iommu_group.  
> 
> This looks good to me.
> 
> > 
> > This also however highlights that our VF backed mdevs will have the
> > same issue, so maybe this new IOMMU API interface should mimic
> > vfio_mdev_attach_domain() more directly, testing whether the resulting
> > device supports IOMMU_DEV_FEAT_AUX and using an aux vs non-aux attach.
> > I'm not sure what the name of this combined function should be,
> > iommu_attach_group_with_lookup()?  This could be the core
> > implementation of iommu_attach_group() where the existing function
> > simply wraps the call with a NULL function pointer.
> > 
> > Anyway, I think there are ways to implement this that are more in line
> > with the spirit of groups.  
> 
> Another possible implementation, just for discussion purpose:
> 
> 1. Add a member in group_device to save the iommu_device if it exists:
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index b6858adc4f17..6474e82cf4b4 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -47,9 +47,16 @@ struct iommu_group {
>          struct list_head entry;
>   };
> 
> +/*
> + * dma_alias: The device put in this group might depends on another
> + *            physical device to do the DMA remapping. At(de)taching
> + *            the domain to/from @dma_alias instead of @dev if
> + *            @dma_alias is set.
> + */
>   struct group_device {
>          struct list_head list;
>          struct device *dev;
> +       struct device *dma_alias;
>          char *name;
>   };
> 
> 2. Pass in the iommu_device when calling iommu_group_add_device().
> 
> int iommu_group_add_device(struct iommu_group *group,
>                             struct device *dev,
>                             struct device *dma_alias)
> 
> Hence, the iommu core could get a chance to set the iommu_device in the
> group device.
> 
> 3. Mimic vfio_mdev_attach_domain() logic in iommu_group_do_attach_device():
> 
> if (group->dma_alias) {
> 	if (iommu_dev_feature_enabled(group->dma_alias, IOMMU_DEV_FEAT_AUX))
> 		iommu_aux_attach_device(domain, group->dma_alias);
> 	else
> 		__iommu_attach_device(domain, group->dma_alias);
> } else {
> 	__iommu_attach_device(domain, dev);
> }
> 
> One limitation is that the driver should call mdev_set_iommu_device()
> before the mdev_probe() get called.

That's an option, but DMA aliases are an existing thing within our
IOMMU/PCI constructs, so I'd steer away from "dma_alias" terminology.  I
thought the callback was a little less invasive to the IOMMU layer for
now as aux domains are still a rather unique use case, and I'm not sure
we can justify the hack of otherwise IOMMU backed mdevs formally within
the IOMMU API.  Thanks,

Alex

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