linux-kernel - RE: [PATCH V2 2/3 net-next] Drivers: hv: vmbus: Fix a bug in the signalling logic with kick

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Date:	Thu, 12 Mar 2015 03:31:50 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: RE: [PATCH V2 2/3 net-next] Drivers: hv: vmbus: Fix a bug in the
 signalling logic with kick_q



> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Wednesday, March 11, 2015 8:08 PM
> To: KY Srinivasan
> Cc: davem@...emloft.net; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; gregkh@...uxfoundation.org; KY Srinivasan
> Subject: Re: [PATCH V2 2/3 net-next] Drivers: hv: vmbus: Fix a bug in the
> signalling logic with kick_q
> 
> 
> 
> On Thu, Mar 12, 2015 at 3:04 AM, K. Y. Srinivasan <kys@...rosoft.com>
> wrote:
> > When the caller specifies that signalling should be deferred, we need
> > to address the case where we are not able to place the current packet
> > because the buffer is full. In this case, we will signal the host as
> > some packets may have been placed on the ring buffer.
> > I would like to thank Jason Wang <jasowang@...hat.com> for pointing
> > out this issue.
> >
> > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > ---
> >  drivers/hv/channel.c |   32 ++++++++++++++++++++++++++++++++
> >  1 files changed, 32 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c index
> > e58cdb7..ae06ba9 100644
> > --- a/drivers/hv/channel.c
> > +++ b/drivers/hv/channel.c
> > @@ -614,8 +614,24 @@ int vmbus_sendpacket_ctl(struct vmbus_channel
> > *channel, void *buffer,
> >
> >  	ret = hv_ringbuffer_write(&channel->outbound, bufferlist, 3,
> > &signal);
> >
> > +	/*
> > +	 * Here is the logic for signalling the host:
> > +	 * 1. If the host is already draining the ringbuffer,
> > +	 *    don't signal. This is indicated by the parameter
> > +	 *    "signal".
> > +	 *
> > +	 * 2. If we are not able to write, signal if kick_q is false.
> > +	 *    kick_q being false indicates that we may have placed zero or
> > +	 *    more packets with more packets to come. We will signal in
> > +	 *    this case even if potentially we may have not placed any
> > +	 *    packet. This is a rare enough condition that it should not
> > +	 *    matter.
> > +	 */
> > +
> >  	if ((ret == 0) && kick_q && signal)
> >  		vmbus_setevent(channel);
> > +	else if ((ret != 0) && !kick_q)
> > +		vmbus_setevent(channel);
> >
> >  	return ret;
> >  }
> > @@ -705,8 +721,24 @@ int vmbus_sendpacket_pagebuffer_ctl(struct
> > vmbus_channel *channel,
> >
> >  	ret = hv_ringbuffer_write(&channel->outbound, bufferlist, 3,
> > &signal);
> >
> > +	/*
> > +	 * Here is the logic for signalling the host:
> > +	 * 1. If the host is already draining the ringbuffer,
> > +	 *    don't signal. This is indicated by the parameter
> > +	 *    "signal".
> > +	 *
> > +	 * 2. If we are not able to write, signal if kick_q is false.
> > +	 *    kick_q being false indicates that we may have placed zero or
> > +	 *    more packets with more packets to come. We will signal in
> > +	 *    this case even if potentially we may have not placed any
> > +	 *    packet. This is a rare enough condition that it should not
> > +	 *    matter.
> > +	 */
> > +
> >  	if ((ret == 0) && kick_q && signal)
> >  		vmbus_setevent(channel);
> > +	else if ((ret != 0) && !kick_q)
> > +		vmbus_setevent(channel);
> >
> >  	return ret;
> >  }
> > --
> 
> Looks like we need to kick unconditionally here. Consider we may get -
> EAGAIN when we want to send the last skb (kick_q is true) from the list. We
> need kick host in this case.
> 
> Btw, another method is let the driver to decide e.g exporting the
> vmbus_setevent() and call it in netvsc_start_xmit().

There is other state that governs if the host needs to be signaled and I don't think we want to 
expose that to netvsc. In any case, netvsc will have to signal the host when EGAIN is received.
We might as well do it in the vmbus driver.

Thank you Jason, I will respin and resubmit the series.

K. Y
> 
> 
>