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From:   "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
To:     Stephen Hemminger <stephen@networkplumber.org>,
        Michael Kelley <mhkelley@outlook.com>
CC:     "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
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        "jasowang@redhat.com" <jasowang@redhat.com>,
        "leann.ogasawara@canonical.com" <leann.ogasawara@canonical.com>,
        "marcelo.cerri@canonical.com" <marcelo.cerri@canonical.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: RE: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer
 signaling
Thread-Topic: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer
 signaling
Thread-Index: AQHTorCNaQB4t9RCq0qElSZLQrwNpaOinHeAgACOaAA=
Date:   Wed, 14 Feb 2018 02:12:08 +0000
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> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Tuesday, February 13, 2018 9:35 AM
> To: Michael Kelley <mhkelley@outlook.com>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@microsoft.com>; gregkh@linuxf=
oundation.org;
> linux-kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.d=
e;
> apw@canonical.com; vkuznets@redhat.com; jasowang@redhat.com;
> leann.ogasawara@canonical.com; marcelo.cerri@canonical.com; Stephen Hemmi=
nger
> <sthemmin@microsoft.com>; KY Srinivasan <kys@microsoft.com>
> Subject: Re: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer si=
gnaling
>=20
>=20
> >  	if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) {
> >  		u32 pending_sz =3D 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 =3D 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 <=3D pending_sz)
> >  			return;
> > +
> > +		vmbus_setevent(channel);
> >  	}
> >
> > -	vmbus_setevent(channel);
> >  }
>=20
> I think this won't work on older versions of Windows where feat_pending_s=
z
> 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