Re: [dpdk-dev] [PATCH v5 4/7] ethdev: copy fast-path API into separate structure

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 10/9/21 3:05 PM, fengchengwen wrote:
> On 2021/10/7 19:27, Konstantin Ananyev wrote:
>> Copy public function pointers (rx_pkt_burst(), etc.) and related
>> pointers to internal data from rte_eth_dev structure into a
>> separate flat array. That array will remain in a public header.
>> The intention here is to make rte_eth_dev and related structures internal.
>> That should allow future possible changes to core eth_dev structures
>> to be transparent to the user and help to avoid ABI/API breakages.
>> The plan is to keep minimal part of data from rte_eth_dev public,
>> so we still can use inline functions for fast-path calls
>> (like rte_eth_rx_burst(), etc.) to avoid/minimize slowdown.
>> The whole idea beyond this new schema:
>> 1. PMDs keep to setup fast-path function pointers and related data
>>    inside rte_eth_dev struct in the same way they did it before.
>> 2. Inside rte_eth_dev_start() and inside rte_eth_dev_probing_finish()
>>    (for secondary process) we call eth_dev_fp_ops_setup, which
>>    copies these function and data pointers into rte_eth_fp_ops[port_id].
>> 3. Inside rte_eth_dev_stop() and inside rte_eth_dev_release_port()
>>    we call eth_dev_fp_ops_reset(), which resets rte_eth_fp_ops[port_id]
>>    into some dummy values.
>> 4. fast-path ethdev API (rte_eth_rx_burst(), etc.) will use that new
>>    flat array to call PMD specific functions.
>> That approach should allow us to make rte_eth_devices[] private
>> without introducing regression and help to avoid changes in drivers code.
>>
>> Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
>> ---
>>  lib/ethdev/ethdev_private.c  | 52 ++++++++++++++++++++++++++++++++++
>>  lib/ethdev/ethdev_private.h  |  7 +++++
>>  lib/ethdev/rte_ethdev.c      | 27 ++++++++++++++++++
>>  lib/ethdev/rte_ethdev_core.h | 55 ++++++++++++++++++++++++++++++++++++
>>  4 files changed, 141 insertions(+)
>>
>> diff --git a/lib/ethdev/ethdev_private.c b/lib/ethdev/ethdev_private.c
>> index 012cf73ca2..3eeda6e9f9 100644
>> --- a/lib/ethdev/ethdev_private.c
>> +++ b/lib/ethdev/ethdev_private.c
>> @@ -174,3 +174,55 @@ rte_eth_devargs_parse_representor_ports(char *str, void *data)
>>  		RTE_LOG(ERR, EAL, "wrong representor format: %s\n", str);
>>  	return str == NULL ? -1 : 0;
>>  }
>> +
>> +static uint16_t
>> +dummy_eth_rx_burst(__rte_unused void *rxq,
>> +		__rte_unused struct rte_mbuf **rx_pkts,
>> +		__rte_unused uint16_t nb_pkts)
>> +{
>> +	RTE_ETHDEV_LOG(ERR, "rx_pkt_burst for unconfigured port\n");
>> +	rte_errno = ENOTSUP;
>> +	return 0;
>> +}
>> +
>> +static uint16_t
>> +dummy_eth_tx_burst(__rte_unused void *txq,
>> +		__rte_unused struct rte_mbuf **tx_pkts,
>> +		__rte_unused uint16_t nb_pkts)
>> +{
>> +	RTE_ETHDEV_LOG(ERR, "tx_pkt_burst for unconfigured port\n");
>> +	rte_errno = ENOTSUP;
>> +	return 0;
>> +}
>> +
>> +void
>> +eth_dev_fp_ops_reset(struct rte_eth_fp_ops *fpo)
> 
> The port_id parameter is preferable, this will hide rte_eth_fp_ops as much as possible.

Sorry, but I see no point to hide it inside ethdev.
Of course, prototype should be reconsidered if we make
it ethdev-internal API available for drivers.
If so, I agree that the parameter should be port_id.

[snip]

>> diff --git a/lib/ethdev/ethdev_private.h b/lib/ethdev/ethdev_private.h
>> index 3724429577..5721be7bdc 100644
>> --- a/lib/ethdev/ethdev_private.h
>> +++ b/lib/ethdev/ethdev_private.h
>> @@ -26,4 +26,11 @@ eth_find_device(const struct rte_eth_dev *_start, rte_eth_cmp_t cmp,
>>  /* Parse devargs value for representor parameter. */
>>  int rte_eth_devargs_parse_representor_ports(char *str, void *data);
>>  
>> +/* reset eth fast-path API to dummy values */
>> +void eth_dev_fp_ops_reset(struct rte_eth_fp_ops *fpo);
>> +
>> +/* setup eth fast-path API to ethdev values */
>> +void eth_dev_fp_ops_setup(struct rte_eth_fp_ops *fpo,
>> +		const struct rte_eth_dev *dev);
> 
> Some drivers control the transmit/receive function during operation. E.g.
> for hns3 driver, when detect reset, primary process will set rx/tx burst to dummy, after
> process reset, primary process will set the correct rx/tx burst. During this process, the
> send and receive threads are still working, but the bursts they call are changed. So:
> 1. it is recommended that trace be deleted from the dummy function.
> 2. public the eth_dev_fp_ops_reset/setup interface for driver usage.

Good point.

[snip]

>> diff --git a/lib/ethdev/rte_ethdev_core.h b/lib/ethdev/rte_ethdev_core.h
>> index 51cd68de94..d5853dff86 100644
>> --- a/lib/ethdev/rte_ethdev_core.h
>> +++ b/lib/ethdev/rte_ethdev_core.h
>> @@ -50,6 +50,61 @@ typedef int (*eth_rx_descriptor_status_t)(void *rxq, uint16_t offset);
>>  typedef int (*eth_tx_descriptor_status_t)(void *txq, uint16_t offset);
>>  /**< @internal Check the status of a Tx descriptor */
>>  
>> +/**
>> + * @internal
>> + * Structure used to hold opaque pointers to internal ethdev Rx/Tx
>> + * queues data.
>> + * The main purpose to expose these pointers at all - allow compiler
>> + * to fetch this data for fast-path ethdev inline functions in advance.
>> + */
>> +struct rte_ethdev_qdata {
>> +	void **data;
>> +	/**< points to array of internal queue data pointers */
>> +	void **clbk;
>> +	/**< points to array of queue callback data pointers */
>> +};
>> +
>> +/**
>> + * @internal
>> + * fast-path ethdev functions and related data are hold in a flat array.
>> + * One entry per ethdev.
>> + * On 64-bit systems contents of this structure occupy exactly two 64B lines.
>> + * On 32-bit systems contents of this structure fits into one 64B line.
>> + */
>> +struct rte_eth_fp_ops {
>> +
>> +	/**
>> +	 * Rx fast-path functions and related data.
>> +	 * 64-bit systems: occupies first 64B line
>> +	 */
>> +	eth_rx_burst_t rx_pkt_burst;
>> +	/**< PMD receive function. */
>> +	eth_rx_queue_count_t rx_queue_count;
>> +	/**< Get the number of used RX descriptors. */
>> +	eth_rx_descriptor_status_t rx_descriptor_status;
>> +	/**< Check the status of a Rx descriptor. */
>> +	struct rte_ethdev_qdata rxq;
>> +	/**< Rx queues data. */
>> +	uintptr_t reserved1[3];
>> +
>> +	/**
>> +	 * Tx fast-path functions and related data.
>> +	 * 64-bit systems: occupies second 64B line
>> +	 */
>> +	eth_tx_burst_t tx_pkt_burst;
> 
> Why not place rx_pkt_burst/tx_pkt_burst/rxq /txq to the first cacheline ?
> Other function, e.g. rx_queue_count/descriptor_status are low frequency call functions.

+1 Very good question
If so, tx_pkt_prepare should be on the first cache-line
as well.

>> +	/**< PMD transmit function. */
>> +	eth_tx_prep_t tx_pkt_prepare;
>> +	/**< PMD transmit prepare function. */
>> +	eth_tx_descriptor_status_t tx_descriptor_status;
>> +	/**< Check the status of a Tx descriptor. */
>> +	struct rte_ethdev_qdata txq;
>> +	/**< Tx queues data. */
>> +	uintptr_t reserved2[3];
>> +
>> +} __rte_cache_aligned;
>> +
>> +extern struct rte_eth_fp_ops rte_eth_fp_ops[RTE_MAX_ETHPORTS];
>> +
>>  
>>  /**
>>   * @internal
>>