netdev - Re: [PATCH] net: orphan queued skbs if device tx can stall

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Message-ID: <20120409082919.GA3471@gondor.apana.org.au>
Date:	Mon, 9 Apr 2012 16:29:20 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <hadi@...erus.ca>,
	Stephen Hemminger <shemminger@...tta.com>,
	Jason Wang <jasowang@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jiri Pirko <jpirko@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH] net: orphan queued skbs if device tx can stall

On Mon, Apr 09, 2012 at 10:39:54AM +0300, Michael S. Tsirkin wrote:
> On Mon, Apr 09, 2012 at 03:33:54PM +0800, Herbert Xu wrote:
> > On Mon, Apr 09, 2012 at 10:28:49AM +0300, Michael S. Tsirkin wrote:
> > >
> > > > 1) Doesn't this break local UDP push-back?
> > > 
> > > What is meant by UDP pushback here? Two tap
> > > devices communicating by UDP packets locally?
> > > This was always broken, see below.
> > 
> > I mean push-back from UDP transmission to the physical NIC.
> > 
> > Your patch breaks that
> 
> I think there's some misunderstanding. pushback is only disabled
> for destinations that set IFF_TX_CAN_STALL. I expect that
> no physical NICs set this flag - only tun and possibly
> other userspace-controlled devices in the future.

I'm talking about an app running in the guest transmitting UDP
to the physical NIC via virtio/vhost.  This will break with your
patch.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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