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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>
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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>