Merge branch 'virtio_net-rx-enable-premapped-mode-by-default'

Xuan Zhuo says:

====================
virtio_net: rx enable premapped mode by default

Actually, for the virtio drivers, we can enable premapped mode whatever
the value of use_dma_api. Because we provide the virtio dma apis.
So the driver can enable premapped mode unconditionally.

This patch set makes the big mode of virtio-net to support premapped mode.
And enable premapped mode for rx by default.

Based on the following points, we do not use page pool to manage these
    pages:

    1. virtio-net uses the DMA APIs wrapped by virtio core. Therefore,
       we can only prevent the page pool from performing DMA operations, and
       let the driver perform DMA operations on the allocated pages.
    2. But when the page pool releases the page, we have no chance to
       execute dma unmap.
    3. A solution to #2 is to execute dma unmap every time before putting
       the page back to the page pool. (This is actually a waste, we don't
       execute unmap so frequently.)
    4. But there is another problem, we still need to use page.dma_addr to
       save the dma address. Using page.dma_addr while using page pool is
       unsafe behavior.
    5. And we need space the chain the pages submitted once to virtio core.

    More:
        https://lore.kernel.org/all/CACGkMEu=Aok9z2imB_c5qVuujSh=vjj1kx12fy9N7hqyi+M5Ow@mail.gmail.com/

Why we do not use the page space to store the dma?

    http://lore.kernel.org/all/CACGkMEuyeJ9mMgYnnB42=hw6umNuo=agn7VBqBqYPd7GN=+39Q@mail.gmail.com
====================

Link: https://lore.kernel.org/r/20240511031404.30903-1-xuanzhuo@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 1 insertions, 6 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 6f7e5010a673..2a972752ff1b 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -2782,7 +2782,7 @@ EXPORT_SYMBOL_GPL(virtqueue_resize);
  *
  * Returns zero or a negative error.
  * 0: success.
- * -EINVAL: vring does not use the dma api, so we can not enable premapped mode.
+ * -EINVAL: too late to enable premapped mode, the vq already contains buffers.
  */
 int virtqueue_set_dma_premapped(struct virtqueue *_vq)
 {
@@ -2798,11 +2798,6 @@ int virtqueue_set_dma_premapped(struct virtqueue *_vq)
 		return -EINVAL;
 	}
 
-	if (!vq->use_dma_api) {
-		END_USE(vq);
-		return -EINVAL;
-	}
-
 	vq->premapped = true;
 	vq->do_unmap = false;