On Tue, 2024-06-04 at 18:17 +0200, Alexander Potapenko wrote:

On Tue, Jun 4, 2024 at 6:07 PM Ilya Leoshkevich [off-list ref]
wrote:

quoted

On Thu, 2023-08-10 at 20:30 +0800, Xuan Zhuo wrote:

quoted

If the vq is the premapped mode, use the sg_dma_address()
directly.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio_ring.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c

b/drivers/virtio/virtio_ring.c
index 8e81b01e0735..f9f772e85a38 100644

--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -361,6 +361,11 @@ static struct device *vring_dma_dev(const

struct
vring_virtqueue *vq)
 static int vring_map_one_sg(const struct vring_virtqueue *vq,
struct
scatterlist *sg,
                          enum dma_data_direction direction,
dma_addr_t *addr)
 {
+     if (vq->premapped) {
+             *addr = sg_dma_address(sg);
+             return 0;
+     }
+

I wonder if something needs to be done for KMSAN here, like it's
done
by the next block in this function? I'm looking into what seems to
be a
KMSAN false positive on s390x:

BUG: KMSAN: uninit-value in receive_buf+0x45ca/0x6990
 receive_buf+0x45ca/0x6990
 virtnet_poll+0x17e0/0x3130
 net_rx_action+0x832/0x26e0
 handle_softirqs+0x330/0x10f0
 [...]

I think there's a similar problem on x86 as well:
https://syzkaller.appspot.com/bug?extid=c5336dcd1b741349d27a

I was going to look closer this week.

Thanks! I bisected it in the meantime and the first failing commit is:

commit f9dac92ba9081062a6477ee015bd3b8c5914efc4
Author: Xuan Zhuo [off-list ref]
Date:   Sat May 11 11:14:01 2024 +0800

    virtio_ring: enable premapped mode whatever use_dma_api

so it's definitely related.