netdev - Re: [PATCH net-next v11 08/12] virtio_net: Use existing classifier if possible

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Message-ID: <e23b94ab-35f6-41fb-91f9-1ba9260fc0ed@nvidia.com>
Date: Wed, 19 Nov 2025 01:18:56 -0600
From: Dan Jurgens <danielj@...dia.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, jasowang@...hat.com, pabeni@...hat.com,
 virtualization@...ts.linux.dev, parav@...dia.com, shshitrit@...dia.com,
 yohadt@...dia.com, xuanzhuo@...ux.alibaba.com, eperezma@...hat.com,
 jgg@...pe.ca, kevin.tian@...el.com, kuba@...nel.org, andrew+netdev@...n.ch,
 edumazet@...gle.com
Subject: Re: [PATCH net-next v11 08/12] virtio_net: Use existing classifier if
 possible

On 11/19/25 12:35 AM, Michael S. Tsirkin wrote:
> On Wed, Nov 19, 2025 at 12:26:23AM -0600, Dan Jurgens wrote:
>> On 11/18/25 3:55 PM, Michael S. Tsirkin wrote:
>>> On Tue, Nov 18, 2025 at 08:38:58AM -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.
>>
>>>> +	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->classifier, &(*c)->classifier, tmp->size)) {
>>>
>>> note that classifier has padding bytes.
>>> comparing these with memcmp is not safe, is it?
>>
>> The reserved bytes are set to 0, this is fine.
> 
> I mean the compiler padding.  set to 0 where?

There's no compiler padding in virtio_net_ff_selector. There are
reserved fields between the count and selector array.

> 
>>>
>>>
>>>> +			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)
>>>
>>> what kind of locking prevents two threads racing in this code?
>>
>> The ethtool calls happen under rtnl_lock.
>>
>>>
>>>
>>>> @@ -6932,29 +6945,30 @@ static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
>>>>  		      (*c)->size);
>>>>  	if (err)
>>>>  		goto err_xarray;
>>>>  
>>>> +	refcount_set(&(*c)->refcount, 1);
>>>
>>>
>>> so you insert uninitialized refcount? can't another thread find it
>>> meanwhile?
>>
>> Again, rtnl_lock.
>>
>>
>>>>  
>>>>  	err = insert_rule(ff, eth_rule, c->id, key, key_size);
>>>>  	if (err) {
>>>>  		/* destroy_classifier will free the classifier */
>>>
>>> will free is no longer correct, is it?
>>
>> Clarified the comment.
>>
>>>
>>>> -		destroy_classifier(ff, c->id);
>>>> +		try_destroy_classifier(ff, c->id);
>>>>  		goto err_key;
>>>>  	}
>>>>  
>>>> -- 
>>>> 2.50.1
>>>
>