Add a memory barrier to DMA functions

The emulated devices can run simultaneously with the guest, so
we need to be careful with ordering of load and stores done by
them to the guest system memory, which need to be observed in
the right order by the guest operating system.

This adds a barrier call to the basic DMA read/write ops which
is currently implemented as a smp_mb(), but could be later
improved for more fine grained control of barriers.

Additionally, a _relaxed() variant of the accessors is provided
to easily convert devices who would be performance sensitive
and negatively impacted by the change.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/dma-helpers.c b/dma-helpers.c
index 2e09ceb912..35cb500581 100644
--- a/dma-helpers.c
+++ b/dma-helpers.c
@@ -31,6 +31,8 @@ static void do_dma_memory_set(dma_addr_t addr, uint8_t c, dma_addr_t len)
 
 int dma_memory_set(DMAContext *dma, dma_addr_t addr, uint8_t c, dma_addr_t len)
 {
+    dma_barrier(dma, DMA_DIRECTION_FROM_DEVICE);
+
     if (dma_has_iommu(dma)) {
         return iommu_dma_memory_set(dma, addr, c, len);
     }