netdev - Re: [PATCH V4 net-next 3/3] tun: rx batching

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Date:   Mon, 9 Jan 2017 10:39:55 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, stephen@...workplumber.org, wexu@...hat.com,
        stefanha@...hat.com
Subject: Re: [PATCH V4 net-next 3/3] tun: rx batching



On 2017年01月07日 03:47, Michael S. Tsirkin wrote:
>> +static int tun_get_coalesce(struct net_device *dev,
>> +			    struct ethtool_coalesce *ec)
>> +{
>> +	struct tun_struct *tun = netdev_priv(dev);
>> +
>> +	ec->rx_max_coalesced_frames = tun->rx_batched;
>> +
>> +	return 0;
>> +}
>> +
>> +static int tun_set_coalesce(struct net_device *dev,
>> +			    struct ethtool_coalesce *ec)
>> +{
>> +	struct tun_struct *tun = netdev_priv(dev);
>> +
>> +	if (ec->rx_max_coalesced_frames > NAPI_POLL_WEIGHT)
>> +		return -EINVAL;
> So what should userspace do? Keep trying until it succeeds?
> I think it's better to just use NAPI_POLL_WEIGHT instead and DTRT here.
>

Well, looking at how set_coalesce is implemented in other drivers, 
-EINVAL is usually used when user give a value that exceeds the 
limitation. For tuntap, what missed here is probably just a 
documentation for coalescing in tuntap.txt. (Or extend ethtool to return 
the max value). This seems much better than silently reduce the value to 
the limitation.

Thanks