linux-kernel - Re: [PATCH 04/11] PM: EM: Add the infrastructure for command processing.

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Message-ID: <aD4CrOmIwhUI-pet@wunner.de>
Date: Mon, 2 Jun 2025 21:59:40 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Changwoo Min <changwoo@...lia.com>
Cc: lukasz.luba@....com, rafael@...nel.org, len.brown@...el.com,
	pavel@...nel.org, christian.loehle@....com, tj@...nel.org,
	kernel-dev@...lia.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/11] PM: EM: Add the infrastructure for command
 processing.

On Thu, May 29, 2025 at 09:13:08AM +0900, Changwoo Min wrote:
> +static int em_genl_cmd_doit(struct sk_buff *skb, struct genl_info *info)
> +{
> +	struct param p = { .attrs = info->attrs };
> +	struct sk_buff *msg;
> +	void *hdr;
> +	int cmd = info->genlhdr->cmd;
> +	int ret = -EMSGSIZE;
> +
> +	msg = genlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL);
> +	if (!msg)
> +		return -ENOMEM;

Just a heads-up, I know everyone recommends NLMSG_GOODSIZE but in reality
it's not that great because netlink_trim() reallocates the skb and copies
the entire linear buffer if it determines that the skb is half-empty.
Performance suffers as a result.  So it's actually better to calculate
the exact message length prior to allocation.  See the SPDM commit
referenced in my previous e-mail.  Another lesson I had to learn the
hard way. :(

Thanks,

Lukas