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

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Message-ID: <2e2d61fa-affd-486a-b724-b458d722304c@nvidia.com>
Date: Tue, 15 Jul 2025 17:08:45 +0000
From: Chaitanya Kulkarni <chaitanyak@...dia.com>
To: Jens Axboe <axboe@...nel.dk>, "axboe@...com" <axboe@...com>
CC: "aurelien.aptel@...il.com" <aurelien.aptel@...il.com>, Shai Malin
	<smalin@...dia.com>, "malin1024@...il.com" <malin1024@...il.com>, Or Gerlitz
	<ogerlitz@...dia.com>, Yoray Zack <yorayz@...dia.com>, Boris Pismenny
	<borisp@...dia.com>, "davem@...emloft.net" <davem@...emloft.net>,
	"kbusch@...nel.org" <kbusch@...nel.org>, "linux-nvme@...ts.infradead.org"
	<linux-nvme@...ts.infradead.org>, Gal Shalom <galshalom@...dia.com>, Max
 Gurtovoy <mgurtovoy@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "gus@...labora.com"
	<gus@...labora.com>, "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, Aurelien Aptel
	<aaptel@...dia.com>, "sagi@...mberg.me" <sagi@...mberg.me>, "hch@....de"
	<hch@....de>, "kuba@...nel.org" <kuba@...nel.org>
Subject: Re: [PATCH v30 03/20] iov_iter: skip copy if src == dst for direct
 data placement

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@...dia.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@...dia.com>
>> Signed-off-by: Boris Pismenny <borisp@...dia.com>
>> Signed-off-by: Or Gerlitz <ogerlitz@...dia.com>
>> Signed-off-by: Yoray Zack <yorayz@...dia.com>
>> Signed-off-by: Shai Malin <smalin@...dia.com>
>> Signed-off-by: Aurelien Aptel <aaptel@...dia.com>
>> Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>
>> Reviewed-by: Max Gurtovoy <mgurtovoy@...dia.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