From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <dev-bounces@dpdk.org>
Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124])
	by inbox.dpdk.org (Postfix) with ESMTP id 302DCA00C4;
	Mon, 11 Apr 2022 10:17:52 +0200 (CEST)
Received: from [217.70.189.124] (localhost [127.0.0.1])
	by mails.dpdk.org (Postfix) with ESMTP id CB22F40685;
	Mon, 11 Apr 2022 10:17:51 +0200 (CEST)
Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182])
 by mails.dpdk.org (Postfix) with ESMTP id E34524067B;
 Mon, 11 Apr 2022 10:17:50 +0200 (CEST)
Received: by linux.microsoft.com (Postfix, from userid 1086)
 id F3DD72058469; Mon, 11 Apr 2022 01:17:49 -0700 (PDT)
DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com F3DD72058469
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com;
 s=default; t=1649665070;
 bh=/U+BYa1vRYeeckRIoQUGMtGG0AkIbDW7cdd/COraj7I=;
 h=Date:From:To:Cc:Subject:References:In-Reply-To:From;
 b=bACxkehE+gzqkkTm9Yoeq958ESY6+LBRPo3eWi7RDpo62n2Vcz0lpZTLstQxSHN7i
 gAxWYPU6AZIsSGxsIt/KarVoX+b5LBCWpnEwNHNyCG8hOEPWJB02Nip8OgYFVp7oT7
 TE2E0CCM2vXabSKPbC9LayIqz7JdyiCFnjgGM4f4=
Date: Mon, 11 Apr 2022 01:17:49 -0700
From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Cc: dev@dpdk.org, stable@dpdk.org, Thomas Monjalon <thomas@monjalon.net>,
 Ferruh Yigit <ferruh.yigit@intel.com>,
 Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Subject: Re: [PATCH] ethdev: prohibit polling of a stopped queue
Message-ID: <20220411081749.GA19395@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
References: <20220410213550.1733330-1-dkozlyuk@nvidia.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220410213550.1733330-1-dkozlyuk@nvidia.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-BeenThere: dev@dpdk.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: DPDK patches and discussions <dev.dpdk.org>
List-Unsubscribe: <https://mails.dpdk.org/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://mails.dpdk.org/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <https://mails.dpdk.org/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
Errors-To: dev-bounces@dpdk.org

On Mon, Apr 11, 2022 at 12:35:50AM +0300, Dmitry Kozlyuk wrote:
> Whether it is allowed to call Rx/Tx functions for a stopped queue
> was undocumented. Some PMDs make this behavior a no-op
> either by explicitly checking the queue state
> or by the way how their routines are implemented or HW works.
> 
> No-op behavior may be convenient for application developers.
> But it also means that pollers of stopped queues
> would go all the way down to PMD Rx/Tx routines, wasting cycles.
> Some PMDs would do a check for the queue state on data path,
> even though it may never be needed for a particular application.
> Also, use cases for stopping queues or starting them deferred
> do not logically require polling stopped queues.
> 
> Use case 1: a secondary that was polling the queue has crashed,
> the primary is doing a recovery to free all mbufs.
> By definition the queue to be restarted is not polled.
> 
> Use case 2: deferred queue start or queue reconfiguration.
> The polling thread must be synchronized anyway,
> because queue start and stop are non-atomic.
> 
> Prohibit calling Rx/Tx functions on stopped queues.
> 
> Fixes: 0748be2cf9a2 ("ethdev: queue start and stop")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>

Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>