netdev - Re: [PATCH net-next v10 2/3] xen networking: add basic XDP support for xen-netfront

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Message-ID: <CAOJe8K28RQuiAKADY2pgad8qAzVXYxYcZnb8m0AJGSZTnAfJqA@mail.gmail.com>
Date:   Mon, 22 Jun 2020 15:45:46 +0300
From:   Denis Kirjanov <kda@...ux-powerpc.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, jgross@...e.com, wei.liu@...nel.org,
        paul@....org, ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next v10 2/3] xen networking: add basic XDP support
 for xen-netfront

On 6/22/20, Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> On Mon, 22 Jun 2020 12:21:11 +0300 Denis Kirjanov <kda@...ux-powerpc.org>
> wrote:
>
>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>> index 482c6c8..1b9f49e 100644
>> --- a/drivers/net/xen-netfront.c
>> +++ b/drivers/net/xen-netfront.c
> [...]
>> @@ -560,6 +572,65 @@ static u16 xennet_select_queue(struct net_device
>> *dev, struct sk_buff *skb,
>>  	return queue_idx;
>>  }
>>
>> +static int xennet_xdp_xmit_one(struct net_device *dev, struct xdp_frame
>> *xdpf)
>> +{
>> +	struct netfront_info *np = netdev_priv(dev);
>> +	struct netfront_stats *tx_stats = this_cpu_ptr(np->tx_stats);
>> +	unsigned int num_queues = dev->real_num_tx_queues;
>> +	struct netfront_queue *queue = NULL;
>> +	struct xen_netif_tx_request *tx;
>> +	unsigned long flags;
>> +	int notify;
>> +
>> +	queue = &np->queues[smp_processor_id() % num_queues];
>> +
>> +	spin_lock_irqsave(&queue->tx_lock, flags);
>
> Why are you taking a lock per packet (xdp_frame)?
Hi Jesper,

We have to protect shared ring indices.

>
>> +
>> +	tx = xennet_make_first_txreq(queue, NULL,
>> +				     virt_to_page(xdpf->data),
>> +				     offset_in_page(xdpf->data),
>> +				     xdpf->len);
>> +
>> +	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&queue->tx, notify);
>> +	if (notify)
>> +		notify_remote_via_irq(queue->tx_irq);
>> +
>> +	u64_stats_update_begin(&tx_stats->syncp);
>> +	tx_stats->bytes += xdpf->len;
>> +	tx_stats->packets++;
>> +	u64_stats_update_end(&tx_stats->syncp);
>> +
>> +	xennet_tx_buf_gc(queue);
>> +
>> +	spin_unlock_irqrestore(&queue->tx_lock, flags);
>
> Is the irqsave/irqrestore variant really needed here?

netpoll also invokes the tx completion handler.

>
>> +	return 0;
>> +}
>> +
>> +static int xennet_xdp_xmit(struct net_device *dev, int n,
>> +			   struct xdp_frame **frames, u32 flags)
>> +{
>> +	int drops = 0;
>> +	int i, err;
>> +
>> +	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
>> +		return -EINVAL;
>> +
>> +	for (i = 0; i < n; i++) {
>> +		struct xdp_frame *xdpf = frames[i];
>> +
>> +		if (!xdpf)
>> +			continue;
>> +		err = xennet_xdp_xmit_one(dev, xdpf);
>> +		if (err) {
>> +			xdp_return_frame_rx_napi(xdpf);
>> +			drops++;
>> +		}
>> +	}
>> +
>> +	return n - drops;
>> +}
>
>
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>
>