> -----Original Message-----
> From: Stephen Hemminger <step...@networkplumber.org>
> Sent: Tuesday, February 13, 2018 9:35 AM
> To: Michael Kelley <mhkel...@outlook.com>
> Cc: Michael Kelley (EOSG) <michael.h.kel...@microsoft.com>; 
> gre...@linuxfoundation.org;
> linux-kernel@vger.kernel.org; de...@linuxdriverproject.org; o...@aepfle.de;
> a...@canonical.com; vkuzn...@redhat.com; jasow...@redhat.com;
> leann.ogasaw...@canonical.com; marcelo.ce...@canonical.com; Stephen Hemminger
> <sthem...@microsoft.com>; KY Srinivasan <k...@microsoft.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

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