netdev - Re: [RFC PATCH] virtio_net: Extend virtio to use VF datapath when available

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Date:   Tue, 19 Dec 2017 20:07:01 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        alexander.duyck@...il.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>
Subject: Re: [RFC PATCH] virtio_net: Extend virtio to use VF datapath when
 available

On Tue, Dec 19, 2017 at 09:55:48AM -0800, Stephen Hemminger wrote:
> On Tue, 19 Dec 2017 09:41:39 -0800
> "Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:
> 
> > On 12/19/2017 7:47 AM, Michael S. Tsirkin wrote:
> > > I'll need to look at this more, in particular the feature
> > > bit is missing here. For now one question:
> > >
> > > On Mon, Dec 18, 2017 at 04:40:36PM -0800, Sridhar Samudrala wrote:  
> > >> @@ -56,6 +58,8 @@ module_param(napi_tx, bool, 0644);
> > >>    */
> > >>   DECLARE_EWMA(pkt_len, 0, 64)
> > >>   
> > >> +#define VF_TAKEOVER_INT	(HZ / 10)
> > >> +
> > >>   #define VIRTNET_DRIVER_VERSION "1.0.0"
> > >>   
> > >>   static const unsigned long guest_offloads[] = {  
> > > Why is this delay necessary? And why by 100ms?  
> > 
> > This is based on netvsc implementation and here is the commit that
> > added this delay.  Not sure if this needs to be 100ms.
> > 
> > commit 6123c66854c174e4982f98195100c1d990f9e5e6
> > Author: stephen hemminger <stephen@...workplumber.org>
> > Date:   Wed Aug 9 17:46:03 2017 -0700
> > 
> >      netvsc: delay setup of VF device
> > 
> >      When VF device is discovered, delay bring it automatically up in
> >      order to allow userspace to some simple changes (like renaming).
> > 
> > 
> > 
> 
> could be 10ms, just enough to let udev do its renaming

Isn't there a way not to depend on udev completing its thing within a given timeframe?