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

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Date:   Tue, 10 Jan 2017 01:12:36 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...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 Mon, Jan 09, 2017 at 10:39:55AM +0800, Jason Wang wrote:
> 
> 
> 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


I don't think it's better, it's mostly that
1. there's a hardware limit so it does not change much
2. default is enabled and no one bothers changing

I don't see how will tuntap.txt help if we want to change it
in the future.

-- 
MST