Subject: Re: [PATCH net-next v12 08/12] virtio_net: Use existing classifier if possible

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 24 Nov 2025 17:04:38 -0500

On Wed, Nov 19, 2025 at 01:15:19PM -0600, Daniel Jurgens wrote:
> Classifiers can be used by more than one rule. If there is an existing
> classifier, use it instead of creating a new one. If duplicate
> classifiers are created it would artifically limit the number of rules
> to the classifier limit, which is likely less than the rules limit.
> 
> Signed-off-by: Daniel Jurgens <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Shahar Shitrit <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>
> ---
> v4:
>     - Fixed typo in commit message
>     - for (int -> for (
> 
> v8:
>     - Removed unused num_classifiers. Jason Wang
> 
> v12:
>     - Clarified comment about destroy_classifier freeing. MST
>     - Renamed the classifier field of virtnet_classifier to obj. MST
>     - Explained why in commit message. MST
> ---
> ---
>  drivers/net/virtio_net.c | 51 ++++++++++++++++++++++++++--------------
>  1 file changed, 34 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7600e2383a72..5e49cd78904f 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -32,6 +32,7 @@
>  #include <uapi/linux/virtio_pci.h>
>  #include <uapi/linux/virtio_net_ff.h>
>  #include <linux/xarray.h>
> +#include <linux/refcount.h>
>  
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
> @@ -302,7 +303,6 @@ struct virtnet_ff {
>  	struct virtio_net_ff_cap_mask_data *ff_mask;
>  	struct virtio_net_ff_actions *ff_actions;
>  	struct xarray classifiers;
> -	int num_classifiers;
>  	struct virtnet_ethtool_ff ethtool;
>  };
>  
> @@ -5816,12 +5816,13 @@ struct virtnet_ethtool_rule {
>  /* The classifier struct must be the last field in this struct */
>  struct virtnet_classifier {
>  	size_t size;
> +	refcount_t refcount;
>  	u32 id;
> -	struct virtio_net_resource_obj_ff_classifier classifier;
> +	struct virtio_net_resource_obj_ff_classifier obj;
>  };
>  
>  static_assert(sizeof(struct virtnet_classifier) ==
> -	      ALIGN(offsetofend(struct virtnet_classifier, classifier),
> +	      ALIGN(offsetofend(struct virtnet_classifier, obj),
>  		    __alignof__(struct virtnet_classifier)),
>  	      "virtnet_classifier: classifier must be the last member");
>  
> @@ -5909,11 +5910,24 @@ static bool validate_mask(const struct virtnet_ff *ff,
>  	return false;
>  }
>  
> -static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
> +static int setup_classifier(struct virtnet_ff *ff,
> +			    struct virtnet_classifier **c)
>  {
> +	struct virtnet_classifier *tmp;
> +	unsigned long i;
>  	int err;
>  
> -	err = xa_alloc(&ff->classifiers, &c->id, c,
> +	xa_for_each(&ff->classifiers, i, tmp) {
> +		if ((*c)->size == tmp->size &&
> +		    !memcmp(&tmp->obj, &(*c)->obj, tmp->size)) {
> +			refcount_inc(&tmp->refcount);
> +			kfree(*c);
> +			*c = tmp;
> +			goto out;
> +		}
> +	}
> +
> +	err = xa_alloc(&ff->classifiers, &(*c)->id, *c,
>  		       XA_LIMIT(0, le32_to_cpu(ff->ff_caps->classifiers_limit) - 1),
>  		       GFP_KERNEL);
>  	if (err)
> @@ -5921,29 +5935,30 @@ static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
>  
>  	err = virtio_admin_obj_create(ff->vdev,
>  				      VIRTIO_NET_RESOURCE_OBJ_FF_CLASSIFIER,
> -				      c->id,
> +				      (*c)->id,
>  				      VIRTIO_ADMIN_GROUP_TYPE_SELF,
>  				      0,
> -				      &c->classifier,
> -				      c->size);
> +				      &(*c)->obj,
> +				      (*c)->size);
>  	if (err)
>  		goto err_xarray;
>  
> +	refcount_set(&(*c)->refcount, 1);
> +out:
>  	return 0;
>  
>  err_xarray:
> -	xa_erase(&ff->classifiers, c->id);
> +	xa_erase(&ff->classifiers, (*c)->id);
>  
>  	return err;
>  }
>  
> -static void destroy_classifier(struct virtnet_ff *ff,
> -			       u32 classifier_id)
> +static void try_destroy_classifier(struct virtnet_ff *ff, u32 classifier_id)
>  {
>  	struct virtnet_classifier *c;
>  
>  	c = xa_load(&ff->classifiers, classifier_id);
> -	if (c) {
> +	if (c && refcount_dec_and_test(&c->refcount)) {
>  		virtio_admin_obj_destroy(ff->vdev,
>  					 VIRTIO_NET_RESOURCE_OBJ_FF_CLASSIFIER,
>  					 c->id,
> @@ -5967,7 +5982,7 @@ static void destroy_ethtool_rule(struct virtnet_ff *ff,
>  				 0);
>  
>  	xa_erase(&ff->ethtool.rules, eth_rule->flow_spec.location);
> -	destroy_classifier(ff, eth_rule->classifier_id);
> +	try_destroy_classifier(ff, eth_rule->classifier_id);
>  	kfree(eth_rule);
>  }
>  
> @@ -6139,7 +6154,7 @@ static int build_and_insert(struct virtnet_ff *ff,
>  	}
>  
>  	c->size = classifier_size;
> -	classifier = &c->classifier;
> +	classifier = &c->obj;
>  	classifier->count = num_hdrs;
>  	selector = (void *)&classifier->selectors[0];
>  
> @@ -6149,14 +6164,16 @@ static int build_and_insert(struct virtnet_ff *ff,
>  	if (err)
>  		goto err_key;
>  
> -	err = setup_classifier(ff, c);
> +	err = setup_classifier(ff, &c);
>  	if (err)
>  		goto err_classifier;
>  
>  	err = insert_rule(ff, eth_rule, c->id, key, key_size);
>  	if (err) {
> -		/* destroy_classifier will free the classifier */
> -		destroy_classifier(ff, c->id);
> +		/* destroy_classifier release the reference on the classifier

try_destroy_classifier ? and I think you mean *will* release and free.

and what is "the reference"

> +		 * and free it if needed.
> +		 */
> +		try_destroy_classifier(ff, c->id);
>  		goto err_key;
>  	}
>  
> -- 
> 2.50.1