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

[Michael Kelley (EOSG)]

> -----Original Message-----
> From: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> Sent: Tuesday, February 13, 2018 9:35 AM
> To: Michael Kelley <mhkelley@xxxxxxxxxxx>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@xxxxxxxxxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> apw@xxxxxxxxxxxxx; vkuznets@xxxxxxxxxx; jasowang@xxxxxxxxxx;
> leann.ogasawara@xxxxxxxxxxxxx; marcelo.cerri@xxxxxxxxxxxxx; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>; KY Srinivasan <kys@xxxxxxxxxxxxx>
> 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