vsock/virtio: MSG_ZEROCOPY flag support

This adds handling of MSG_ZEROCOPY flag on transmission path:

1) If this flag is set and zerocopy transmission is possible (enabled
   in socket options and transport allows zerocopy), then non-linear
   skb will be created and filled with the pages of user's buffer.
   Pages of user's buffer are locked in memory by 'get_user_pages()'.
2) Replaces way of skb owning: instead of 'skb_set_owner_sk_safe()' it
   calls 'skb_set_owner_w()'. Reason of this change is that
   '__zerocopy_sg_from_iter()' increments 'sk_wmem_alloc' of socket, so
   to decrease this field correctly, proper skb destructor is needed:
   'sock_wfree()'. This destructor is set by 'skb_set_owner_w()'.
3) Adds new callback to 'struct virtio_transport': 'can_msgzerocopy'.
   If this callback is set, then transport needs extra check to be able
   to send provided number of buffers in zerocopy mode. Currently, the
   only transport that needs this callback set is virtio, because this
   transport adds new buffers to the virtio queue and we need to check,
   that number of these buffers is less than size of the queue (it is
   required by virtio spec). vhost and loopback transports don't need
   this check.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index a91fbdf233e4..ebb3ce63d64d 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -160,6 +160,15 @@ struct virtio_transport {
 
 	/* Takes ownership of the packet */
 	int (*send_pkt)(struct sk_buff *skb);
+
+	/* Used in MSG_ZEROCOPY mode. Checks, that provided data
+	 * (number of buffers) could be transmitted with zerocopy
+	 * mode. If this callback is not implemented for the current
+	 * transport - this means that this transport doesn't need
+	 * extra checks and can perform zerocopy transmission by
+	 * default.
+	 */
+	bool (*can_msgzerocopy)(int bufs_num);
 };
 
 ssize_t