From: Konrad Rzeszutek Wilk Date: Fri, 20 Nov 2015 17:22:14 +0000 (-0500) Subject: libvchan: Read prod/cons only once. X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=7d66a4ba695ab8d13b214fb816dd59e443ae1ec9;p=people%2Fliuw%2Flibxenctrl-split%2Fxen.git 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 --- diff --git a/tools/libvchan/io.c b/tools/libvchan/io.c index 8a9629ba88..381cc05172 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. */