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Date:   Tue, 13 Feb 2018 09:34:39 -0800
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     Michael Kelley <mhkelley@outlook.com>
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        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
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        "sthemmin@microsoft.com" <sthemmin@microsoft.com>,
        "kys@microsoft.com" <kys@microsoft.com>
Subject: Re: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer
 signaling
Message-ID: <20180213093439.3698cb5e@xeon-e3>
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>  	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.