Re: [PATCH v4 06/16] iommu/arm-smmu-v3: Hold arm_smmu_asid_lock during all of attach_dev

[Mostafa Saleh <smostafa@google.com>]

Hi Jason,

On Thu, Jan 25, 2024 at 07:57:16PM -0400, Jason Gunthorpe wrote:
> The BTM support wants to be able to change the ASID of any smmu_domain.
> When it goes to do this it holds the arm_smmu_asid_lock and iterates over
> the target domain's devices list.
> 
> During attach of a S1 domain we must ensure that the devices list and
> CD are in sync, otherwise we could miss CD updates or a parallel CD update
> could push an out of date CD.
> 
> This is pretty complicated, and almost works today because
> arm_smmu_detach_dev() removes the master from the linked list before
> working on the CD entries, preventing parallel update of the CD.
> 
> However, it does have an issue where the CD can remain programed while the
> domain appears to be unattached. arm_smmu_share_asid() will then not clear
> any CD entriess and install its own CD entry with the same ASID
> concurrently. This creates a small race window where the IOMMU can see two
> ASIDs pointing to different translations.

I don’t see the race condition.

The current flow is as follows,
For SVA, if the asid was used by domain_x, it will do:

lock(arm_smmu_asid_lock)
Alloc new asid and set cd->asid.
lock(domain_x->devices_lock)
Write new CD with the new asid
unlock(domain_x->devices_lock)
unlock(arm_smmu_asid_lock)

For attach_dev (domain_y), if the device was attached to domain_z
//Detach old domain
lock(domain_z->devices_lock)
Remove master from old domain
unlock(domain_z->devices_lock)
Clear CD
//Attach new domain
lock(arm_smmu_asid_lock)
Allocate ASID
unlock(arm_smmu_asid_lock)

lock(domain_y->devices_lock)
Insert new master.
unlock(domain_y->devices_lock)

lock(arm_smmu_asid_lock)
Write CD
unlock(arm_smmu_asid_lock)


In case
1) domain_x == domain_z(old domain)
Write to the CD is protected by domain_x->devices_lock, so either:
    a) The device will be removed, so SVA code will not touch it, and the
    detach will clear the CD.
    b) The device CD will be updated from the SVA with the new code, but then
    it will be removed from the domain and cleared.

I don’t see any case where we end with a programmed CD.

2) domain_x == domain_y(new domain)

Similarly the device would either see the new CD(new asid) or the old CD
then the new CD.

Can you please clarify the race condition? as it seems I am missing something.

> Solve this by wrapping most of the attach flow in the
> arm_smmu_asid_lock. This locks more than strictly needed to prepare for
> the next patch which will reorganize the order of the linked list, STE and
> CD changes.
> 
> Move arm_smmu_detach_dev() till after we have initialized the domain so
> the lock can be held for less time.
> 
> Reviewed-by: Michael Shavit <mshavit@google.com>
> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Tested-by: Moritz Fischer <moritzf@google.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 22 ++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 9a95d0f1494223..539ef380f457fa 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -2612,8 +2612,6 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  		return -EBUSY;
>  	}
>  
> -	arm_smmu_detach_dev(master);
> -
>  	mutex_lock(&smmu_domain->init_mutex);
>  
>  	if (!smmu_domain->smmu) {
> @@ -2628,6 +2626,16 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  	if (ret)
>  		return ret;
>  
> +	/*
> +	 * Prevent arm_smmu_share_asid() from trying to change the ASID
> +	 * of either the old or new domain while we are working on it.
> +	 * This allows the STE and the smmu_domain->devices list to
> +	 * be inconsistent during this routine.
> +	 */
> +	mutex_lock(&arm_smmu_asid_lock);
> +
> +	arm_smmu_detach_dev(master);
> +
>  	master->domain = smmu_domain;
>  
>  	/*
> @@ -2653,13 +2661,7 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  			}
>  		}
>  
> -		/*
> -		 * Prevent SVA from concurrently modifying the CD or writing to
> -		 * the CD entry
> -		 */
> -		mutex_lock(&arm_smmu_asid_lock);
>  		ret = arm_smmu_write_ctx_desc(master, IOMMU_NO_PASID, &smmu_domain->cd);
> -		mutex_unlock(&arm_smmu_asid_lock);
>  		if (ret) {
>  			master->domain = NULL;
>  			goto out_list_del;
> @@ -2669,13 +2671,15 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
>  	arm_smmu_install_ste_for_dev(master);
>  
>  	arm_smmu_enable_ats(master);
> -	return 0;
> +	goto out_unlock;
>  
>  out_list_del:
>  	spin_lock_irqsave(&smmu_domain->devices_lock, flags);
>  	list_del(&master->domain_head);
>  	spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
>  
> +out_unlock:
> +	mutex_unlock(&arm_smmu_asid_lock);
>  	return ret;
>  }
>  
> -- 
> 2.43.0
> 

Thanks,
Mostafa