Re: [PATCH vhost v13 04/12] virtio_ring: support add premapped buf

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On Tue, 04 Jun 2024 18:07:44 +0200, Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote:
> On Thu, 2023-08-10 at 20:30 +0800, Xuan Zhuo wrote:
> > If the vq is the premapped mode, use the sg_dma_address() directly.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > ---
> >  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
>  [...]
>
> Uninit was created at:
>  __alloc_pages_noprof+0x62a/0xe60
>  alloc_pages_noprof+0x392/0x830
>  skb_page_frag_refill+0x21a/0x5c0
>  virtnet_rq_alloc+0x50/0x1500
>  try_fill_recv+0x372/0x54c0
>  virtnet_open+0x210/0xbe0
>  __dev_open+0x56e/0x920
>  __dev_change_flags+0x39c/0x2000
>  dev_change_flags+0xaa/0x200
>  do_setlink+0x197a/0x7420
>  rtnl_setlink+0x77c/0x860
>  [...]
>
> My understanding is that virtnet_rq_alloc() allocates a page for
> receiving data from a virtio device, which is then wrapped in struct
> scatterlist by virtnet_rq_init_one_sg(), which is in turn associated
> with a virtqueue through the virtqueue_add_inbuf_ctx() ->
> virtqueue_add() -> virtqueue_add_split() -> vring_map_one_sg()
> call chain.
>
> Someone should unpoison this page (since KMSAN doesn't know that the
> hypervisor writes to it), and today for the non-premapped case this is
> vring_map_one_sg(). So I tried the following naive fix:
>
>         if (vq->premapped) {
>                 *addr = sg_dma_address(sg);
> +               if (!vq->use_dma_api) {
> +                       kmsan_handle_dma(phys_to_page(*addr), sg-
> >offset, sg->length, direction);
> +               }
>
> but it didn't help. I plan to investigate this further, but any hints
> are much appreciated.
>
> >  	if (!vq->use_dma_api) {
> >  		/*
> >  		 * If DMA is not used, KMSAN doesn't know that the
> > scatterlist


Could you try this?

Thanks.

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 37c9c5b55864..cb280b66c7a2 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -3119,8 +3119,10 @@ dma_addr_t virtqueue_dma_map_single_attrs(struct virtqueue *_vq, void *ptr,
 {
        struct vring_virtqueue *vq = to_vvq(_vq);

-       if (!vq->use_dma_api)
+       if (!vq->use_dma_api) {
+               kmsan_handle_dma(virt_to_page(ptr), offset_in_page(ptr), size, dir);
                return (dma_addr_t)virt_to_phys(ptr);
+       }

        return dma_map_single_attrs(vring_dma_dev(vq), ptr, size, dir, attrs);
 }
@@ -3171,8 +3173,10 @@ dma_addr_t virtqueue_dma_map_page_attrs(struct virtqueue *_vq, struct page *page
 {
        struct vring_virtqueue *vq = to_vvq(_vq);

-       if (!vq->use_dma_api)
+       if (!vq->use_dma_api) {
+               kmsan_handle_dma(page, offset, size, dir);
                return page_to_phys(page) + offset;
+       }

        return dma_map_page_attrs(vring_dma_dev(vq), page, offset, size, dir, attrs);
 }





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