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

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Message-ID: <e80a7828-f42f-4263-89c1-66512b2dde6e@nvidia.com>
Date: Wed, 19 Nov 2025 01:33:31 -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 1:23 AM, Michael S. Tsirkin wrote:
> On Wed, Nov 19, 2025 at 01:18:56AM -0600, Dan Jurgens wrote:
>> 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.
> 
> I might be missing something here, but are not the
> structures this code compares of the type struct virtnet_classifier
> not virtio_net_ff_selector ?
> 
> and that one is:
> 
>  struct virtnet_classifier {
>         size_t size;
> +       refcount_t refcount;
>         u32 id;
>         struct virtio_net_resource_obj_ff_classifier classifier;
>  };
> 
> 
> which seems to have some padding depending on the architecture.

We're only comparing the ->classifier part of that, which is pad free.

> 
> 
>