linux-kernel - RE: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling

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Date:   Wed, 14 Feb 2018 02:12:08 +0000
From:   "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Michael Kelley <mhkelley@...look.com>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.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>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "leann.ogasawara@...onical.com" <leann.ogasawara@...onical.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>
Subject: RE: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer
 signaling

> -----Original Message-----
> From: Stephen Hemminger <stephen@...workplumber.org>
> Sent: Tuesday, February 13, 2018 9:35 AM
> To: Michael Kelley <mhkelley@...look.com>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@...rosoft.com>; gregkh@...uxfoundation.org;
> linux-kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; vkuznets@...hat.com; jasowang@...hat.com;
> leann.ogasawara@...onical.com; marcelo.cerri@...onical.com; Stephen Hemminger
> <sthemmin@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>
> Subject: Re: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling
> 
> 
> >  	if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) {
> >  		u32 pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
> >
> >  		/*
> > +		 * Ensure the read of write_index in hv_get_bytes_to_write()
> > +		 * happens after the read of pending_send_sz.
> > +		 */
> > +		virt_rmb();
> > +		curr_write_sz = hv_get_bytes_to_write(rbi);
> > +
> > +		/*
> >  		 * If there was space before we began iteration,
> >  		 * then host was not blocked. Also handles case where
> >  		 * pending_sz is zero then host has nothing pending
> >  		 * and does not need to be signaled.
> >  		 */
> > -		if (orig_write_sz > pending_sz)
> > +		if (curr_write_sz - delta > pending_sz)
> >  			return;
> >
> >  		/* If pending write will not fit, don't give false hope. */
> > -		if (hv_get_bytes_to_write(rbi) < pending_sz)
> > +		if (curr_write_sz <= pending_sz)
> >  			return;
> > +
> > +		vmbus_setevent(channel);
> >  	}
> >
> > -	vmbus_setevent(channel);
> >  }
> 
> I think this won't work on older versions of Windows where feat_pending_sz
> is never set.  On those versions vmbus_setevent needs to always be called.

The older Windows/Hyper-V hosts poll if they can't write to the
host->guest ring buffer because it is full.   We don't want to call
vmbus_setevent() every time through this routine because then
the guest would be signaling the host every time the host interrupts
the guest.

Michael