From c1fce65e2b720684ea6ba76ae59921542bd154bb Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Fri, 20 Nov 2015 12:22:14 -0500 Subject: [PATCH 3/3] 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 --- tools/libvchan/io.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/tools/libvchan/io.c b/tools/libvchan/io.c index 8a9629b..381cc05 100644 --- a/tools/libvchan/io.c +++ b/tools/libvchan/io.c @@ -117,6 +117,7 @@ static inline int send_notify(struct libxenvchan *ctrl, uint8_t bit) 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. */ @@ -158,6 +159,7 @@ int libxenvchan_data_ready(struct libxenvchan *ctrl) 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. */ -- 2.1.0