netdev - RE: [PATCH ethtool-next v4 2/2] netlink: add netlink handler for get rss (-x)

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Date:   Mon, 9 Jan 2023 18:07:45 +0000
From:   "Mogilappagari, Sudheer" <sudheer.mogilappagari@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
Subject: RE: [PATCH ethtool-next v4 2/2] netlink: add netlink handler for get
 rss (-x)



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Friday, January 6, 2023 1:42 PM
> To: Mogilappagari, Sudheer <sudheer.mogilappagari@...el.com>
> Cc: netdev@...r.kernel.org; mkubecek@...e.cz; andrew@...n.ch;
> Samudrala, Sridhar <sridhar.samudrala@...el.com>; Nguyen, Anthony L
> <anthony.l.nguyen@...el.com>
> Subject: Re: [PATCH ethtool-next v4 2/2] netlink: add netlink handler
> for get rss (-x)
> 
> On Fri, 6 Jan 2023 17:41:39 +0000 Mogilappagari, Sudheer wrote:
> > > > +	rss_hfunc = mnl_attr_get_u32(tb[ETHTOOL_A_RSS_HFUNC]);
> > > > +
> > > > +	indir_bytes =
> mnl_attr_get_payload_len(tb[ETHTOOL_A_RSS_INDIR]);
> > > > +	indir_table = mnl_attr_get_str(tb[ETHTOOL_A_RSS_INDIR]);
> > > > +
> > > > +	hkey_bytes =
> mnl_attr_get_payload_len(tb[ETHTOOL_A_RSS_HKEY]);
> > > > +	hkey = mnl_attr_get_str(tb[ETHTOOL_A_RSS_HKEY]);
> > >
> > > All elements of tb can be NULL, AFAIU.
> >
> > Didn't get this. Do you mean the variables need to be checked for
> NULL
> > here? If yes, am checking before printing later on.
> 
> tb[ETHTOOL_A_RSS_HKEY] can be NULL. I just realized now that the kernel
> always fills in the attrs:
> 
> 	if (nla_put_u32(skb, ETHTOOL_A_RSS_HFUNC, data->hfunc) ||
> 	    nla_put(skb, ETHTOOL_A_RSS_INDIR,
> 		    sizeof(u32) * data->indir_size, data->indir_table) ||
> 	    nla_put(skb, ETHTOOL_A_RSS_HKEY, data->hkey_size, data-
> >hkey))
> 		return -EMSGSIZE;
> 
> but that's not a great use of netlink. If there is nothing to report
> the attribute should simply be skipped, like this:
> 
> 	if (nla_put_u32(skb, ETHTOOL_A_RSS_HFUNC, data->hfunc)) ||
> 	    (data->indir_size &&
> 	     nla_put(skb, ETHTOOL_A_RSS_INDIR,
> 		     sizeof(u32) * data->indir_size, data->indir_table))
> ||
> 	    (data->hkey_size &&
> 	     nla_put(skb, ETHTOOL_A_RSS_HKEY, data->hkey_size, data-
> >hkey)))
> 		return -EMSGSIZE;
> 
> I think we should fix the kernel side.

Will do this. 

> 
> > > > +int get_channels_cb(const struct nlmsghdr *nlhdr, void *data) {
> >
> > > > +	silent = nlctx->is_dump || nlctx->is_monitor;
> > > > +	err_ret = silent ? MNL_CB_OK : MNL_CB_ERROR;
> > > > +	ret = mnl_attr_parse(nlhdr, GENL_HDRLEN, attr_cb,
> &tb_info);
> > > > +	if (ret < 0)
> > > > +		return err_ret;
> > > > +	nlctx->devname =
> get_dev_name(tb[ETHTOOL_A_CHANNELS_HEADER]);
> > >
> > > We need to check that the kernel has filled in the attrs before
> > > accessing them, no?
> >
> > Didn't get this one either. similar code isn't doing any checks like
> > you suggested.
> 
> Same point, really. Even if the kernel always populates the attribute
> today, according to netlink rules it's not guaranteed to do so in the
> future, so any tb[] access should be NULL-checked.
> 
> > > The correct value is combined + rx, I think I mentioned this
> before.
> >
> > Have changed it to include rx too like below.
> > args->num_rings =
> > args->mnl_attr_get_u32(tb[ETHTOOL_A_CHANNELS_COMBINED_COUNT]);
> > args->num_rings += mnl_attr_get_u32(tb[ETHTOOL_A_CHANNELS_RX_COUNT]);
> 
> Related to previous comments - take a look at channels_fill_reply().
> tb[ETHTOOL_A_CHANNELS_RX_COUNT] will be NULL for most devices.
> mnl_attr_get_u32(NULL) will crash.
> 
> > Slightly unrelated, where can I find the reason behind using combined
> + rx?
> > Guess it was discussed in mailing list but not able to find it.
> 
> Yes, it's been discussed on the list multiple times but man ethtool is
> the only source of documentation I know of.
> 
> The reason is that the channels API refers to interrupts more than
> queues. So rx means an _irq_ dedicated to Rx processing, not an Rx
> queue. Unfortunately the author came up with the term "channel" which
> most people take to mean "queue" :(
> 
> > > +	return MNL_CB_OK;
> > >
> > > I'm also not sure if fetching the channel info shouldn't be done
> > > over the nl2 socket, like the string set. If we are in monitor mode
> > > we may confuse ourselves trying to parse the wrong messages.
> >
> > Are you suggesting we need to use ioctl for fetching ring info to
> > avoid mix-up. Is there alternative way to do it ?
> 
> No no, look how the strings for hfunc names are fetched - they are
> fetched over a different socket, right?

global_stringset is using nlctx->ethnl2_socket. Are you suggesting use
of it for fetching channels info too ? 

ret = netlink_init_ethnl2_socket(nlctx);
...
hash_funcs = global_stringset(ETH_SS_RSS_HASH_FUNCS, nlctx->ethnl2_socket);