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

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

[Michael Kelley]

Fix bugs in signaling the Hyper-V host when freeing space in the
host->guest ring buffer:

1. The interrupt_mask must not be used to determine whether to signal
   on the host->guest ring buffer
2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
   *after* pending_send_sz is read in order to avoid a race condition
3. Comparisons with pending_send_sz must treat the "equals" case as
   not-enough-space
4. Don't signal if the pending_send_sz feature is not present. Older
   versions of Hyper-V that don't implement this feature will poll.

Fixes: 03bad714a161 ("vmbus: more host signalling avoidance")
Signed-off-by: Michael Kelley <mhkelley@outlook.com>
---
 drivers/hv/ring_buffer.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 50e0714..b64be18 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -423,7 +423,11 @@ struct vmpacket_descriptor *
 void hv_pkt_iter_close(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
-	u32 orig_write_sz = hv_get_bytes_to_write(rbi);
+	u32 curr_write_sz;
+	u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ?
+			(rbi->priv_read_index - rbi->ring_buffer->read_index) :
+			(rbi->ring_datasize - rbi->ring_buffer->read_index +
+			rbi->priv_read_index);
 
 	/*
 	 * Make sure all reads are done before we update the read index since
@@ -446,27 +450,31 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 */
 	virt_mb();
 
-	/* If host has disabled notifications then skip */
-	if (rbi->ring_buffer->interrupt_mask)
-		return;
-
 	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);
 }
 EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
-- 
1.8.3.1

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

[Stephen Hemminger]

On Sat, 10 Feb 2018 20:48:49 +0000
Michael Kelley <mhkelley@outlook.com> wrote:

> +	u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ?
> +			(rbi->priv_read_index - rbi->ring_buffer->read_index) :
> +			(rbi->ring_datasize - rbi->ring_buffer->read_index +
> +			rbi->priv_read_index);
>  

Would it would make sense for this to be a wrapper function like other ring operations?
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RE: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling

[KY Srinivasan]

> -----Original Message-----
> From: Michael Kelley [mailto:mhkelley@outlook.com]
> Sent: Saturday, February 10, 2018 12:49 PM
> To: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> vkuznets@redhat.com; jasowang@redhat.com;
> leann.ogasawara@canonical.com; marcelo.cerri@canonical.com; Stephen
> Hemminger <sthemmin@microsoft.com>; KY Srinivasan
> <kys@microsoft.com>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@microsoft.com>
> Subject: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling
> 
> Fix bugs in signaling the Hyper-V host when freeing space in the
> host->guest ring buffer:
> 
> 1. The interrupt_mask must not be used to determine whether to signal
>    on the host->guest ring buffer
> 2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
>    *after* pending_send_sz is read in order to avoid a race condition
> 3. Comparisons with pending_send_sz must treat the "equals" case as
>    not-enough-space
> 4. Don't signal if the pending_send_sz feature is not present. Older
>    versions of Hyper-V that don't implement this feature will poll.
> 
> Fixes: 03bad714a161 ("vmbus: more host signalling avoidance")
> Signed-off-by: Michael Kelley <mhkelley@outlook.com>
> ---
>  drivers/hv/ring_buffer.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 50e0714..b64be18 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -423,7 +423,11 @@ struct vmpacket_descriptor *
>  void hv_pkt_iter_close(struct vmbus_channel *channel)
>  {
>  	struct hv_ring_buffer_info *rbi = &channel->inbound;
> -	u32 orig_write_sz = hv_get_bytes_to_write(rbi);
> +	u32 curr_write_sz;
> +	u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ?
> +			(rbi->priv_read_index - rbi->ring_buffer-
> >read_index) :
> +			(rbi->ring_datasize - rbi->ring_buffer->read_index +
> +			rbi->priv_read_index);
> 
>  	/*
>  	 * Make sure all reads are done before we update the read index
> since
> @@ -446,27 +450,31 @@ void hv_pkt_iter_close(struct vmbus_channel
> *channel)
>  	 */
>  	virt_mb();
> 
> -	/* If host has disabled notifications then skip */
> -	if (rbi->ring_buffer->interrupt_mask)
> -		return;
> -
>  	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();
We can avoid the read barrier by making the initialization of curr_write_sz conditional
on pending_send_sz being non-zero. Indeed you can make all the signaling code conditional
on pending_send_sz being non-zero.



> +		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);
>  }
>  EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
> --
> 1.8.3.1

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

[Michael Kelley (EOSG)]

> -----Original Message-----
> From: KY Srinivasan
> Sent: Sunday, February 11, 2018 5:14 PM

--- snip ---

> >  	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();
> We can avoid the read barrier by making the initialization of curr_write_sz conditional
> on pending_send_sz being non-zero. Indeed you can make all the signaling code conditional
> on pending_send_sz being non-zero.
> 
> 

I agree that we can immediately test pending_send_sz for zero, and exit
if zero.  A  zero value will be by far the most common path, and would
save executing the read barrier and hv_get_bytes_to_write().  Can also
move the calculation of "delta" to after the test for zero.

But I believe the read barrier is still needed on the path where
pending_send_sz is non-zero.  Just the testing the value of pending_send_sz
doesn't guarantee that the write_index wouldn't be speculatively read,
and potentially out-of-order.   And we have to consider the out-of-order
behaviors on ARM64 as well.

> 
> > +		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);
> >  }
> >  EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
> > --
> > 1.8.3.1

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

[Stephen Hemminger]

>  	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.

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

[Michael Kelley (EOSG)]

> -----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@linuxfoundation.org;
> linux-kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> apw@canonical.com; vkuznets@redhat.com; jasowang@redhat.com;
> leann.ogasawara@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
> 
> 
> >  	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

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

[Stephen Hemminger]

What about the following simplification. If on older hosts you can avoid more code
and the additional barrier. Also cleanup comment wording somewhat.



>From de2d566d9092cbbc8e5974dea581617ef787ff69 Mon Sep 17 00:00:00 2001
From: Michael Kelley <mhkelley@outlook.com>
Date: Sat, 10 Feb 2018 20:48:49 +0000
Subject: [PATCH 1/9] Drivers: hv: vmbus: Fix ring buffer signaling

Fix bugs in signaling the Hyper-V host when freeing space in the
host->guest ring buffer:

1. The interrupt_mask must not be used to determine whether to signal
   on the host->guest ring buffer
2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
   *after* pending_send_sz is read in order to avoid a race condition
3. Comparisons with pending_send_sz must treat the "equals" case as
   not-enough-space
4. Don't signal if the pending_send_sz feature is not present. Older
   versions of Hyper-V that don't implement this feature will poll.

Fixes: 03bad714a161 ("vmbus: more host signalling avoidance")
Signed-off-by: Michael Kelley <mhkelley@outlook.com>
---
 drivers/hv/ring_buffer.c | 67 +++++++++++++++++++++++++++++-------------------
 1 file changed, 40 insertions(+), 27 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 50e071444a5c..d27cbb66279c 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -417,13 +417,25 @@ __hv_pkt_iter_next(struct vmbus_channel *channel,
 }
 EXPORT_SYMBOL_GPL(__hv_pkt_iter_next);
 
+/* How many bytes were read in this iterator cycle */
+static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi)
+{
+	if (rbi->ring_buffer->read_index < rbi->priv_read_index)
+		return rbi->priv_read_index - rbi->ring_buffer->read_index;
+	else
+		return rbi->ring_datasize -
+			rbi->ring_buffer->read_index + rbi->priv_read_index;
+}
+
 /*
  * Update host ring buffer after iterating over packets.
  */
 void hv_pkt_iter_close(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
-	u32 orig_write_sz = hv_get_bytes_to_write(rbi);
+	u32 curr_write_sz;
+	u32 delta = hv_pkt_iter_bytes_read(rbi);
+	u32 pending_sz;
 
 	/*
 	 * Make sure all reads are done before we update the read index since
@@ -433,39 +445,40 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	virt_rmb();
 	rbi->ring_buffer->read_index = rbi->priv_read_index;
 
+	/* Older versions do not require signalling */
+	if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
+		return;
+
 	/*
-	 * Issue a full memory barrier before making the signaling decision.
-	 * Here is the reason for having this barrier:
-	 * If the reading of the pend_sz (in this function)
-	 * were to be reordered and read before we commit the new read
-	 * index (in the calling function)  we could
-	 * have a problem. If the host were to set the pending_sz after we
-	 * have sampled pending_sz and go to sleep before we commit the
-	 * read index, we could miss sending the interrupt. Issue a full
-	 * memory barrier to address this.
+	 * If the reading of the pend_sz were to be reordered and read
+	 * before we commit the new read index then we would have a
+	 * problem. If the host were to set the pending_sz after we
+	 * have sampled pending_sz and go to sleep before we commit
+	 * the read index, we could miss sending the interrupt.
 	 */
 	virt_mb();
 
-	/* If host has disabled notifications then skip */
-	if (rbi->ring_buffer->interrupt_mask)
-		return;
+	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
 
-	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)
-			return;
+	/*
+	 * 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 (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)
-			return;
-	}
+	/* If pending write will not fit, don't give false hope. */
+	if (curr_write_sz <= pending_sz)
+		return;
 
 	vmbus_setevent(channel);
 }
-- 
2.15.1

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