Subject: Re: [PATCH v6 02/25] iommu/ioasid: Add ioasid references

From: Jacob Pan <[email-masked]>
Date: Thu, 30 Apr 2020 11:39:31 -0700

On Thu, 30 Apr 2020 16:34:01 +0200
Jean-Philippe Brucker <[email-masked]> wrote:

> Let IOASID users take references to existing ioasids with
> ioasid_get(). ioasid_free() drops a reference and only frees the
> ioasid when its reference number is zero. It returns whether the
> ioasid was freed.
> 
Looks good to me, I was planning to do the same for VT-d use. Just a
couple of points for potential extension. I can rebase on top of this.

> Signed-off-by: Jean-Philippe Brucker <[email-masked]>
> ---
>  include/linux/ioasid.h | 10 ++++++++--
>  drivers/iommu/ioasid.c | 30 +++++++++++++++++++++++++++++-
>  2 files changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> index 6f000d7a0ddcd..609ba6f15b9e3 100644
> --- a/include/linux/ioasid.h
> +++ b/include/linux/ioasid.h
> @@ -34,7 +34,8 @@ struct ioasid_allocator_ops {
>  #if IS_ENABLED(CONFIG_IOASID)
>  ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t
> max, void *private);
> -void ioasid_free(ioasid_t ioasid);
> +void ioasid_get(ioasid_t ioasid);
> +bool ioasid_free(ioasid_t ioasid);
>  void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
>  		  bool (*getter)(void *));
>  int ioasid_register_allocator(struct ioasid_allocator_ops
> *allocator); @@ -48,10 +49,15 @@ static inline ioasid_t
> ioasid_alloc(struct ioasid_set *set, ioasid_t min, return
> INVALID_IOASID; }
>  
> -static inline void ioasid_free(ioasid_t ioasid)
> +static inline void ioasid_get(ioasid_t ioasid)
>  {
>  }
>  
> +static inline bool ioasid_free(ioasid_t ioasid)
> +{
> +	return false;
> +}
> +
>  static inline void *ioasid_find(struct ioasid_set *set, ioasid_t
> ioasid, bool (*getter)(void *))
>  {
> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> index 0f8dd377aada3..46511ac53e0c8 100644
> --- a/drivers/iommu/ioasid.c
> +++ b/drivers/iommu/ioasid.c
> @@ -15,6 +15,7 @@ struct ioasid_data {
>  	struct ioasid_set *set;
>  	void *private;
>  	struct rcu_head rcu;
> +	refcount_t refs;
>  };
>  
>  /*
> @@ -314,6 +315,7 @@ ioasid_t ioasid_alloc(struct ioasid_set *set,
> ioasid_t min, ioasid_t max, 
>  	data->set = set;
>  	data->private = private;
> +	refcount_set(&data->refs, 1);
>  
>  	/*
>  	 * Custom allocator needs allocator data to perform platform
> specific @@ -345,12 +347,33 @@ ioasid_t ioasid_alloc(struct
> ioasid_set *set, ioasid_t min, ioasid_t max, }
>  EXPORT_SYMBOL_GPL(ioasid_alloc);
>  
> +/**
> + * ioasid_get - obtain a reference to the IOASID
> + */
> +void ioasid_get(ioasid_t ioasid)
why void? what if the ioasid is not valid.

> +{
> +	struct ioasid_data *ioasid_data;
> +
> +	spin_lock(&ioasid_allocator_lock);
> +	ioasid_data = xa_load(&active_allocator->xa, ioasid);
> +	if (ioasid_data)
> +		refcount_inc(&ioasid_data->refs);
> +	spin_unlock(&ioasid_allocator_lock);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_get);
> +
>  /**
>   * ioasid_free - Free an IOASID
>   * @ioasid: the ID to remove
> + *
> + * Put a reference to the IOASID, free it when the number of
> references drops to
> + * zero.
> + *
> + * Return: %true if the IOASID was freed, %false otherwise.
>   */
> -void ioasid_free(ioasid_t ioasid)
> +bool ioasid_free(ioasid_t ioasid)
>  {
> +	bool free = false;
>  	struct ioasid_data *ioasid_data;
>  
>  	spin_lock(&ioasid_allocator_lock);
> @@ -360,6 +383,10 @@ void ioasid_free(ioasid_t ioasid)
>  		goto exit_unlock;
>  	}
>  
> +	free = refcount_dec_and_test(&ioasid_data->refs);
> +	if (!free)
> +		goto exit_unlock;
> +
Just FYI, we may need to add states for the IOASID, i.g. mark the IOASID
inactive after free. And prohibit ioasid_get() after freed. For VT-d,
this is useful when KVM queries the IOASID.

>  	active_allocator->ops->free(ioasid,
> active_allocator->ops->pdata); /* Custom allocator needs additional
> steps to free the xa element */ if (active_allocator->flags &
> IOASID_ALLOCATOR_CUSTOM) { @@ -369,6 +396,7 @@ void
> ioasid_free(ioasid_t ioasid) 
>  exit_unlock:
>  	spin_unlock(&ioasid_allocator_lock);
> +	return free;
>  }
>  EXPORT_SYMBOL_GPL(ioasid_free);
>  

[Jacob Pan]

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