Re: [PATCH net-next v4 8/9] tap: Keep hdr_len in tap_get_user()

[Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>]

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@xxxxxxxxxx>
> > ---
> >   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.