RE: [EXTERNAL] Re: [PATCH 4/4] net/netvsc: cache device parameters for hot plug events

[Long Li <longli@microsoft.com>]

> Subject: [EXTERNAL] Re: [PATCH 4/4] net/netvsc: cache device parameters for
> hot plug events
> 
> On Mon, 27 Jan 2025 17:35:06 -0800
> longli@linuxonhyperv.com wrote:
> 
> > @@ -1409,9 +1424,6 @@ eth_hn_dev_uninit(struct rte_eth_dev *eth_dev)
> >  	ret_stop = hn_dev_stop(eth_dev);
> >  	hn_dev_close(eth_dev);
> >
> > -	free(hv->vf_devargs);
> > -	hv->vf_devargs = NULL;
> > -
> >  	hn_detach(hv);
> >  	hn_chim_uninit(eth_dev);
> >  	rte_vmbus_chan_close(hv->channels[0]);
> > @@ -1423,6 +1435,61 @@ eth_hn_dev_uninit(struct rte_eth_dev *eth_dev)
> >  	return ret_stop;
> >  }
> >
> > +static int populate_cache_list(void)
> > +{
> > +	int ret;
> > +	struct rte_devargs *da;
> > +
> > +	rte_spinlock_lock(&netvsc_lock);
> > +	da_cache_usage++;
> > +	if (da_cache_usage > 1) {
> > +		ret = 0;
> > +		goto out;
> > +	}
> > +
> > +	LIST_INIT(&da_cache_list);
> > +	RTE_EAL_DEVARGS_FOREACH("pci", da) {
> > +		struct da_cache *cache;
> > +
> > +		cache = rte_zmalloc("NETVSC-HOTADD", sizeof(*cache),
> rte_mem_page_size());
> > +		if (!cache) {
> > +			ret = -ENOMEM;
> > +			goto out;
> > +		}
> > +
> > +		strncpy(cache->name, da->name, sizeof(da->name));
> > +		cache->drv_str = strdup(da->drv_str);
> > +		if (!cache->drv_str) {
> > +			rte_free(cache);
> > +			ret = -ENOMEM;
> > +			goto out;
> > +		}
> > +
> > +		LIST_INSERT_HEAD(&da_cache_list, cache, list);
> > +	}
> 
> Why do you need to cache entry to be page aligned, that seems unnecessary
> wasteful?

You are right it doesn't need to. I'm removing the alignment.

> Why does it need to be huge pages? versus normal malloc?

I thought it would make it easier to debug memory leaks through EAL. But normal malloc is fine because this data is not shared between primary/secondary.

> The string is coming from malloc (strdup) so it can't be used by secondary process.
> 
> Since you are allocating a devargs cache entry, you could just as well use flexible
> array where entry was like:
> struct da_cache {
> 	LIST_ENTRY(da_cache) list;
> 	char name[RTE_DEV_NAME_MAX_LEN];
> 	char drv_str[];
> };

This is difficult as I don't know how big it is for drv_str[].

> 
> 
> 	cache = malloc(sizeof(*cache) + strlen(da->drv_str) + 1); ...
> 	strcpy(cache->drv_str, da->drv_str);

Another approach is to modify EAL to never delete driver arguments when a device is removed. i.e., It doesn't call rte_devargs_remove() on device removal, instead keep those devargs for the lifetime of the process. Do you think this is a better approach? This will save work if other drivers want to cache devargs list for device hot plug events.

Thanks,
Long