linux-kernel - Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get

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Message-ID: <67a3d6706c01a_170d3929436@willemb.c.googlers.com.notmuch>
Date: Wed, 05 Feb 2025 16:21:52 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>, 
 Jonathan Corbet <corbet@....net>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 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, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get_user()

Akihiko Odaki wrote:
> hdr_len is repeatedly used so keep it in a local variable.
> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>

> @@ -682,11 +683,8 @@ 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;

I forgot earlier: this can also use single line statement

    copylen = max(copylen, ETH_HLEN);

And perhaps easiest to follow is

    copylen = hdr_len ?: GOODCOPY_LEN;
    copylen = min(copylen, good_linear);
    copylen = max(copylen, ETH_HLEN);

>  		linear = copylen;
>  		i = *from;
> @@ -697,11 +695,9 @@ 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;

Same