linux-kernel - Re: [PATCH v3 4/4] vfio/type1: Use iommu_aux_at(de)tach

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Date:   Thu, 30 Jul 2020 10:41:32 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        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 v3 4/4] vfio/type1: Use iommu_aux_at(de)tach_group() APIs

Hi Alex,

On 7/30/20 4:32 AM, Alex Williamson wrote:
> On Tue, 14 Jul 2020 13:57:03 +0800
> Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> 
>> Replace iommu_aux_at(de)tach_device() with iommu_aux_at(de)tach_group().
>> It also saves the IOMMU_DEV_FEAT_AUX-capable physcail device in the
>> vfio_group data structure so that it could be reused in other places.
>>
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 44 ++++++---------------------------
>>   1 file changed, 7 insertions(+), 37 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 5e556ac9102a..f8812e68de77 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -100,6 +100,7 @@ struct vfio_dma {
>>   struct vfio_group {
>>   	struct iommu_group	*iommu_group;
>>   	struct list_head	next;
>> +	struct device		*iommu_device;
>>   	bool			mdev_group;	/* An mdev group */
>>   	bool			pinned_page_dirty_scope;
>>   };
>> @@ -1627,45 +1628,13 @@ static struct device *vfio_mdev_get_iommu_device(struct device *dev)
>>   	return NULL;
>>   }
>>   
>> -static int vfio_mdev_attach_domain(struct device *dev, void *data)
>> -{
>> -	struct iommu_domain *domain = data;
>> -	struct device *iommu_device;
>> -
>> -	iommu_device = vfio_mdev_get_iommu_device(dev);
>> -	if (iommu_device) {
>> -		if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
>> -			return iommu_aux_attach_device(domain, iommu_device);
>> -		else
>> -			return iommu_attach_device(domain, iommu_device);
>> -	}
>> -
>> -	return -EINVAL;
>> -}
>> -
>> -static int vfio_mdev_detach_domain(struct device *dev, void *data)
>> -{
>> -	struct iommu_domain *domain = data;
>> -	struct device *iommu_device;
>> -
>> -	iommu_device = vfio_mdev_get_iommu_device(dev);
>> -	if (iommu_device) {
>> -		if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
>> -			iommu_aux_detach_device(domain, iommu_device);
>> -		else
>> -			iommu_detach_device(domain, iommu_device);
>> -	}
>> -
>> -	return 0;
>> -}
>> -
>>   static int vfio_iommu_attach_group(struct vfio_domain *domain,
>>   				   struct vfio_group *group)
>>   {
>>   	if (group->mdev_group)
>> -		return iommu_group_for_each_dev(group->iommu_group,
>> -						domain->domain,
>> -						vfio_mdev_attach_domain);
>> +		return iommu_aux_attach_group(domain->domain,
>> +					      group->iommu_group,
>> +					      group->iommu_device);
> 
> No, we previously iterated all devices in the group and used the aux
> interface only when we have an iommu_device supporting aux.  If we
> simply assume an mdev group only uses an aux domain we break existing
> users, ex. SR-IOV VF backed mdevs.  Thanks,

Oh, yes. Sorry! I didn't consider the physical device backed mdevs
cases.

Looked into this part of code, it seems that there's a lock issue here.
The group->mutex is held in iommu_group_for_each_dev() and will be
acquired again in iommu_attach_device().

How about making it like:

static int vfio_iommu_attach_group(struct vfio_domain *domain,
                                    struct vfio_group *group)
{
         if (group->mdev_group) {
                 struct device *iommu_device = group->iommu_device;

                 if (WARN_ON(!iommu_device))
                         return -EINVAL;

                 if (iommu_dev_feature_enabled(iommu_device, 
IOMMU_DEV_FEAT_AUX))
                         return iommu_aux_attach_device(domain->domain, 
iommu_device);
                 else
                         return iommu_attach_device(domain->domain, 
iommu_device);
         } else {
                 return iommu_attach_group(domain->domain, 
group->iommu_group);
         }
}

The caller (vfio_iommu_type1_attach_group) has guaranteed that all mdevs
in an iommu group should be derived from a same physical device.

Any thoughts?

> 
> Alex

Best regards,
baolu

> 
> 
>>   	else
>>   		return iommu_attach_group(domain->domain, group->iommu_group);
>>   }
>> @@ -1674,8 +1643,8 @@ static void vfio_iommu_detach_group(struct vfio_domain *domain,
>>   				    struct vfio_group *group)
>>   {
>>   	if (group->mdev_group)
>> -		iommu_group_for_each_dev(group->iommu_group, domain->domain,
>> -					 vfio_mdev_detach_domain);
>> +		iommu_aux_detach_group(domain->domain, group->iommu_group,
>> +				       group->iommu_device);
>>   	else
>>   		iommu_detach_group(domain->domain, group->iommu_group);
>>   }
>> @@ -2007,6 +1976,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>>   			return 0;
>>   		}
>>   
>> +		group->iommu_device = iommu_device;
>>   		bus = iommu_device->bus;
>>   	}
>>   
>