[PATCH 2/4] Drivers: hv: vmbus: Annotate accesses to the shared ring buffer indices

[Kameron Carr]

The ring buffer read/write indices live in a page shared with the host,
so the compiler must not split, merge or refetch accesses to them. Add
READ_ONCE()/WRITE_ONCE() in hv_set_next_write_location(),
hv_pkt_iter_close(), hv_get_bytes_to_read() and hv_get_bytes_to_write().
The accesses in hv_ringbuffer_get_debuginfo() are left to the next
patch.

Drop hv_get_ring_bufferindices(). It has one caller and is a one-line
expression on the write index. Inlining it moves the access to the call
site, so hv_ringbuffer_write() can reuse its validated snapshot of that
index, old_write, rather than reading the shared memory a second time.

No functional change intended for a well-behaved host.

Signed-off-by: Kameron Carr <kameroncarr@xxxxxxxxxxxxxxxxxxx>
---
 drivers/hv/ring_buffer.c | 15 ++++-----------
 include/linux/hyperv.h   |  4 ++--
 2 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index a18b309..7f466c5 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -75,7 +75,7 @@ static inline void
 hv_set_next_write_location(struct hv_ring_buffer_info *ring_info,
 		     u32 next_write_location)
 {
-	ring_info->ring_buffer->write_index = next_write_location;
+	WRITE_ONCE(ring_info->ring_buffer->write_index, next_write_location);
 }
 
 /* Get the size of the ring buffer. */
@@ -85,13 +85,6 @@ hv_get_ring_buffersize(const struct hv_ring_buffer_info *ring_info)
 	return ring_info->ring_datasize;
 }
 
-/* Get the read and write indices as u64 of the specified ring buffer. */
-static inline u64
-hv_get_ring_bufferindices(struct hv_ring_buffer_info *ring_info)
-{
-	return (u64)ring_info->ring_buffer->write_index << 32;
-}
-
 /*
  * Helper routine to copy from source to ring buffer.
  * Assume there is enough room. Handles wrap-around in dest case only!!
@@ -358,7 +351,7 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
 		*trans_id = __trans_id;
 
 	/* Set previous packet start */
-	prev_indices = hv_get_ring_bufferindices(outring_info);
+	prev_indices = (u64)old_write << 32;
 
 	next_write_location = hv_copyto_ringbuffer(outring_info,
 					     next_write_location,
@@ -582,8 +575,8 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 * is updated.
 	 */
 	virt_rmb();
-	start_read_index = rbi->ring_buffer->read_index;
-	rbi->ring_buffer->read_index = rbi->priv_read_index;
+	start_read_index = READ_ONCE(rbi->ring_buffer->read_index);
+	WRITE_ONCE(rbi->ring_buffer->read_index, rbi->priv_read_index);
 
 	/*
 	 * Older versions of Hyper-V (before WS2102 and Win8) do not
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 9e109d9..5c65820 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -214,7 +214,7 @@ static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi)
 	u32 read_loc, write_loc, dsize, read;
 
 	dsize = rbi->ring_datasize;
-	read_loc = rbi->ring_buffer->read_index;
+	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
 
 	read = write_loc >= read_loc ? (write_loc - read_loc) :
@@ -229,7 +229,7 @@ static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi)
 
 	dsize = rbi->ring_datasize;
 	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
-	write_loc = rbi->ring_buffer->write_index;
+	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
 
 	write = write_loc >= read_loc ? dsize - (write_loc - read_loc) :
 		read_loc - write_loc;

-- 
2.45.4