linux-kernel - Re: [PATCH net-next v4 8/9] tap: Keep hdr_len in tap_get

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Message-ID: <806def7d-16f3-4d53-abc8-7d18e8c22dcb@daynix.com>
Date: Tue, 21 Jan 2025 14:44:39 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jonathan Corbet <corbet@....net>, Jason Wang <jasowang@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com,
 devel@...nix.com
Subject: Re: [PATCH net-next v4 8/9] tap: Keep hdr_len in tap_get_user()

On 2025/01/20 20:24, Willem de Bruijn wrote:
> Akihiko Odaki wrote:
>> hdr_len is repeatedly used so keep it in a local variable.
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
>> ---
>>   drivers/net/tap.c | 17 +++++++----------
>>   1 file changed, 7 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index 061c2f27dfc83f5e6d0bea4da0e845cc429b1fd8..7ee2e9ee2a89fd539b087496b92d2f6198266f44 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -645,6 +645,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   	int err;
>>   	struct virtio_net_hdr vnet_hdr = { 0 };
>>   	int vnet_hdr_len = 0;
>> +	int hdr_len = 0;
>>   	int copylen = 0;
>>   	int depth;
>>   	bool zerocopy = false;
>> @@ -672,6 +673,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   		err = -EINVAL;
>>   		if (tap16_to_cpu(q, vnet_hdr.hdr_len) > iov_iter_count(from))
>>   			goto err;
>> +		hdr_len = tap16_to_cpu(q, vnet_hdr.hdr_len);
>>   	}
>>   
>>   	len = iov_iter_count(from);
>> @@ -683,11 +685,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   	if (msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
>>   		struct iov_iter i;
>>   
>> -		copylen = vnet_hdr.hdr_len ?
>> -			tap16_to_cpu(q, vnet_hdr.hdr_len) : GOODCOPY_LEN;
>> -		if (copylen > good_linear)
>> -			copylen = good_linear;
>> -		else if (copylen < ETH_HLEN)
>> +		copylen = min(hdr_len ? hdr_len : GOODCOPY_LEN, good_linear);
>> +		if (copylen < ETH_HLEN)
>>   			copylen = ETH_HLEN;
>>   		linear = copylen;
>>   		i = *from;
>> @@ -698,11 +697,9 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   
>>   	if (!zerocopy) {
>>   		copylen = len;
>> -		linear = tap16_to_cpu(q, vnet_hdr.hdr_len);
>> -		if (linear > good_linear)
>> -			linear = good_linear;
>> -		else if (linear < ETH_HLEN)
>> -			linear = ETH_HLEN;
>> +		linear = min(hdr_len, good_linear);
>> +		if (copylen < ETH_HLEN)
>> +			copylen = ETH_HLEN;
> 
> Similar to previous patch, I don't think this patch is significant
> enough to warrant the code churn.

The following patch will require replacing
tap16_to_cpu(q, vnet_hdr.hdr_len)
with
tap16_to_cpu(q->flags, vnet_hdr.hdr_len)

It will make some lines a bit too long. Calling tap16_to_cpu() at 
multiple places is also not good to keep the vnet implementation unified 
as the function inspects vnet_hdr.

This patch is independently too trivial, but I think it is a worthwhile 
cleanup combined with the following patch.