Re: [PATCH v30 03/20] iov_iter: skip copy if src == dst for direct data placement

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

Jens,

On 7/15/25 08:06, Jens Axboe wrote:
> On 7/15/25 7:27 AM, Aurelien Aptel wrote:
>> From: Ben Ben-Ishay <benishay@nvidia.com>
>>
>> When using direct data placement (DDP) the NIC could write the payload
>> directly into the destination buffer and constructs SKBs such that
>> they point to this data. To skip copies when SKB data already resides
>> in the destination buffer we check if (src == dst), and skip the copy
>> when it's true.
>>
>> Signed-off-by: Ben Ben-Ishay <benishay@nvidia.com>
>> Signed-off-by: Boris Pismenny <borisp@nvidia.com>
>> Signed-off-by: Or Gerlitz <ogerlitz@nvidia.com>
>> Signed-off-by: Yoray Zack <yorayz@nvidia.com>
>> Signed-off-by: Shai Malin <smalin@nvidia.com>
>> Signed-off-by: Aurelien Aptel <aaptel@nvidia.com>
>> Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
>> Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>> ---
>>   lib/iov_iter.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
>> index f9193f952f49..47fdb32653a2 100644
>> --- a/lib/iov_iter.c
>> +++ b/lib/iov_iter.c
>> @@ -62,7 +62,14 @@ static __always_inline
>>   size_t memcpy_to_iter(void *iter_to, size_t progress,
>>   		      size_t len, void *from, void *priv2)
>>   {
>> -	memcpy(iter_to, from + progress, len);
>> +	/*
>> +	 * When using direct data placement (DDP) the hardware writes
>> +	 * data directly to the destination buffer, and constructs
>> +	 * IOVs such that they point to this data.
>> +	 * Thus, when the src == dst we skip the memcpy.
>> +	 */
>> +	if (!(IS_ENABLED(CONFIG_ULP_DDP) && iter_to == from + progress))
>> +		memcpy(iter_to, from + progress, len);
>>   	return 0;
>>   }
> This seems like entirely the wrong place to apply this logic...
>

do you have any specific preference where it needs to be moved ?
or any other way you would prefer ?

-ck