File xsa155-xen-0003-libvchan-Read-prod-cons-only-once.patch of Package xen.6121

References: bsc#957988

From ef86ad0b60fe179b1a6fa390e05c339fb44b9cc9 Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Fri, 20 Nov 2015 12:22:14 -0500
Subject: [PATCH] libvchan: Read prod/cons only once.

We must ensure that the prod/cons are only read once and that
the compiler won't try to optimize the reads. That is split
the read of these in multiple instructions influencing later
branch code. As such insert barriers when fetching the cons
and prod index.

This is part of XSA155.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 tools/libvchan/io.c | 2 ++
 1 file changed, 2 insertions(+)

Index: xen-4.4.3-testing/tools/libvchan/io.c
===================================================================
--- xen-4.4.3-testing.orig/tools/libvchan/io.c
+++ xen-4.4.3-testing/tools/libvchan/io.c
@@ -118,6 +118,7 @@ static inline int send_notify(struct lib
 static inline int raw_get_data_ready(struct libxenvchan *ctrl)
 {
 	uint32_t ready = rd_prod(ctrl) - rd_cons(ctrl);
+	xen_mb(); /* Ensure 'ready' is read only once. */
 	if (ready >= rd_ring_size(ctrl))
 		/* We have no way to return errors.  Locking up the ring is
 		 * better than the alternatives. */
@@ -159,6 +160,7 @@ int libxenvchan_data_ready(struct libxen
 static inline int raw_get_buffer_space(struct libxenvchan *ctrl)
 {
 	uint32_t ready = wr_ring_size(ctrl) - (wr_prod(ctrl) - wr_cons(ctrl));
+	xen_mb(); /* Ensure 'ready' is read only once. */
 	if (ready > wr_ring_size(ctrl))
 		/* We have no way to return errors.  Locking up the ring is
 		 * better than the alternatives. */