linux-kernel - Re: [PATCH V2 net-next 2/2] vhost-net: reduce one userspace copy when building XDP buff

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Message-ID: <20250627174825.667e1e5f@kernel.org>
Date: Fri, 27 Jun 2025 17:48:25 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jason Wang <jasowang@...hat.com>
Cc: willemdebruijn.kernel@...il.com, andrew+netdev@...n.ch,
 davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
 mst@...hat.com, eperezma@...hat.com, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH V2 net-next 2/2] vhost-net: reduce one userspace copy
 when building XDP buff

On Thu, 26 Jun 2025 10:14:45 +0800 Jason Wang wrote:
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -690,13 +690,13 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>  	if (unlikely(!buf))
>  		return -ENOMEM;
>  
> -	copied = copy_from_iter(buf, sock_hlen, from);
> -	if (copied != sock_hlen) {
> +	copied = copy_from_iter(buf + pad - sock_hlen, len, from);
> +	if (copied != len) {
>  		ret = -EFAULT;
>  		goto err;
>  	}
>  
> -	gso = buf;
> +	gso = buf + pad - sock_hlen;
>  
>  	if (!sock_hlen)
>  		memset(buf, 0, pad);
> @@ -715,12 +715,8 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>  		}
>  	}
>  
> -	len -= sock_hlen;

we used to adjust @len here, now we don't..

> -	copied = copy_from_iter(buf + pad, len, from);
> -	if (copied != len) {
> -		ret = -EFAULT;
> -		goto err;
> -	}
> +	/* pad contains sock_hlen */
> +	memcpy(buf, buf + pad - sock_hlen, sock_hlen);
>  
>  	xdp_init_buff(xdp, buflen, NULL);
>  	xdp_prepare_buff(xdp, buf, pad, len, true);

.. yet we still use len as the packet size here.
-- 
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