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Drivers: hv: vmbus: Use *_ONCE() to access shared ring buffer indices

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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.

Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
This commit is contained in:
Kameron Carr 2026-10-19 16:09:47 -07:00 committed by Wei Liu
parent 1d287c17cc
commit 10e2eba7cc
2 changed files with 6 additions and 13 deletions
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15
drivers/hv/ring_buffer.c
View File

@ -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

4
include/linux/hyperv.h
View File

@ -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;