linux-kernel - Re: [RFC 13/20] iommu: Extend iommu_at[de]tach

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Date:   Thu, 14 Oct 2021 16:24:03 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Liu Yi L <yi.l.liu@...el.com>
Cc:     alex.williamson@...hat.com, jgg@...dia.com, hch@....de,
        jasowang@...hat.com, joro@...tes.org, jean-philippe@...aro.org,
        kevin.tian@...el.com, parav@...lanox.com, lkml@...ux.net,
        pbonzini@...hat.com, lushenming@...wei.com, eric.auger@...hat.com,
        corbet@....net, ashok.raj@...el.com, yi.l.liu@...ux.intel.com,
        jun.j.tian@...el.com, hao.wu@...el.com, dave.jiang@...el.com,
        jacob.jun.pan@...ux.intel.com, kwankhede@...dia.com,
        robin.murphy@....com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
        linux-kernel@...r.kernel.org, baolu.lu@...ux.intel.com,
        nicolinc@...dia.com
Subject: Re: [RFC 13/20] iommu: Extend iommu_at[de]tach_device() for multiple
 devices group

On Sun, Sep 19, 2021 at 02:38:41PM +0800, Liu Yi L wrote:
> From: Lu Baolu <baolu.lu@...ux.intel.com>
> 
> These two helpers could be used when 1) the iommu group is singleton,
> or 2) the upper layer has put the iommu group into the secure state by
> calling iommu_device_init_user_dma().
> 
> As we want the iommufd design to be a device-centric model, we want to
> remove any group knowledge in iommufd. Given that we already have
> iommu_at[de]tach_device() interface, we could extend it for iommufd
> simply by doing below:
> 
>  - first device in a group does group attach;
>  - last device in a group does group detach.
> 
> as long as the group has been put into the secure context.
> 
> The commit <426a273834eae> ("iommu: Limit iommu_attach/detach_device to
> device with their own group") deliberately restricts the two interfaces
> to single-device group. To avoid the conflict with existing usages, we
> keep this policy and put the new extension only when the group has been
> marked for user_dma.

I still kind of hate this interface because it means an operation that
appears to be explicitly on a single device has an implicit effect on
other devices.

> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/iommu.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index bffd84e978fb..b6178997aef1 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -47,6 +47,7 @@ struct iommu_group {
>  	struct list_head entry;
>  	unsigned long user_dma_owner_id;
>  	refcount_t owner_cnt;
> +	refcount_t attach_cnt;
>  };
>  
>  struct group_device {
> @@ -1994,7 +1995,7 @@ static int __iommu_attach_device(struct iommu_domain *domain,
>  int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
>  {
>  	struct iommu_group *group;
> -	int ret;
> +	int ret = 0;
>  
>  	group = iommu_group_get(dev);
>  	if (!group)
> @@ -2005,11 +2006,23 @@ int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
>  	 * change while we are attaching
>  	 */
>  	mutex_lock(&group->mutex);
> -	ret = -EINVAL;
> -	if (iommu_group_device_count(group) != 1)
> +	if (group->user_dma_owner_id) {
> +		if (group->domain) {
> +			if (group->domain != domain)
> +				ret = -EBUSY;
> +			else
> +				refcount_inc(&group->attach_cnt);
> +
> +			goto out_unlock;
> +		}
> +	} else if (iommu_group_device_count(group) != 1) {

With this condition in the else, how can you ever attach the first
device of a multi-device group?

> +		ret = -EINVAL;
>  		goto out_unlock;
> +	}
>  
>  	ret = __iommu_attach_group(domain, group);
> +	if (!ret && group->user_dma_owner_id)
> +		refcount_set(&group->attach_cnt, 1);
>  
>  out_unlock:
>  	mutex_unlock(&group->mutex);
> @@ -2261,7 +2274,10 @@ void iommu_detach_device(struct iommu_domain *domain, struct device *dev)
>  		return;
>  
>  	mutex_lock(&group->mutex);
> -	if (iommu_group_device_count(group) != 1) {
> +	if (group->user_dma_owner_id) {
> +		if (!refcount_dec_and_test(&group->attach_cnt))
> +			goto out_unlock;
> +	} else if (iommu_group_device_count(group) != 1) {

Shouldn't this path (detach a thing that's not attached), be a no-op
regardless of whether it's a singleton group or not?  Why does one
deserve a warning and one not?

>  		WARN_ON(1);
>  		goto out_unlock;
>  	}
> @@ -3368,6 +3384,7 @@ static int iommu_group_init_user_dma(struct iommu_group *group,
>  
>  	group->user_dma_owner_id = owner;
>  	refcount_set(&group->owner_cnt, 1);
> +	refcount_set(&group->attach_cnt, 0);
>  
>  	/* default domain is unsafe for user-initiated dma */
>  	if (group->domain == group->default_domain)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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