[PATCH v2 11/15] virtio-rng: fix DMA alignment for data buffer

Michael S. Tsirkin
2026-01-05T08:23:40Z

The data buffer in struct virtrng_info is used for DMA_FROM_DEVICE via
virtqueue_add_inbuf() and shares cachelines with the adjacent
CPU-written fields (data_avail, data_idx).

The device writing to the DMA buffer and the CPU writing to adjacent
fields could corrupt each other's data on non-cache-coherent platforms.

Add __dma_from_device_group_begin()/end() annotations to place these
in distinct cache lines.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/char/hw_random/virtio-rng.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index dd998f4fe4f2..eb80a031c7be 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -11,6 +11,7 @@
#include <linux/spinlock.h>
#include <linux/virtio.h>
#include <linux/virtio_rng.h>
+#include <linux/dma-mapping.h>
#include <linux/module.h>
#include <linux/slab.h>

@@ -28,11 +29,13 @@ struct virtrng_info {
unsigned int data_avail;
unsigned int data_idx;
/* minimal size returned by rng_buffer_size() */
+ __dma_from_device_group_begin();
#if SMP_CACHE_BYTES < 32
u8 data[32];
#else
u8 data[SMP_CACHE_BYTES];
#endif
+ __dma_from_device_group_end();
};

static void random_recv_done(struct virtqueue *vq)
--
MST