diff mbox series

[net,v2,3/5] virtio-net: execute xdp_do_flush() before napi_complete_done()

Message ID 20230125074901.2737-4-magnus.karlsson@gmail.com (mailing list archive)
State Accepted
Commit ad7e615f646c9b5b2cf655cdfb9d91a28db4f25a
Delegated to: Netdev Maintainers
Headers show
Series net: xdp: execute xdp_do_flush() before napi_complete_done() | expand

Checks

Context Check Description
netdev/tree_selection success Clearly marked for net
netdev/fixes_present success Fixes tag present in non-next series
netdev/subject_prefix success Link
netdev/cover_letter success Series has a cover letter
netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 0 this patch: 0
netdev/cc_maintainers warning 4 maintainers not CCed: hawk@kernel.org john.fastabend@gmail.com virtualization@lists.linux-foundation.org edumazet@google.com
netdev/build_clang success Errors and warnings before: 0 this patch: 0
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success Fixes tag looks correct
netdev/build_allmodconfig_warn success Errors and warnings before: 0 this patch: 0
netdev/checkpatch success total: 0 errors, 0 warnings, 0 checks, 16 lines checked
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0

Commit Message

Magnus Karlsson Jan. 25, 2023, 7:48 a.m. UTC 
From: Magnus Karlsson <magnus.karlsson@intel.com>

Make sure that xdp_do_flush() is always executed before
napi_complete_done(). This is important for two reasons. First, a
redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
napi context X on CPU Y will be followed by a xdp_do_flush() from the
same napi context and CPU. This is not guaranteed if the
napi_complete_done() is executed before xdp_do_flush(), as it tells
the napi logic that it is fine to schedule napi context X on another
CPU. Details from a production system triggering this bug using the
veth driver can be found following the first link below.

The second reason is that the XDP_REDIRECT logic in itself relies on
being inside a single NAPI instance through to the xdp_do_flush() call
for RCU protection of all in-kernel data structures. Details can be
found in the second link below.

Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
---
 drivers/net/virtio_net.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Comments

Michael S. Tsirkin Jan. 27, 2023, 10:49 a.m. UTC | #1 
On Wed, Jan 25, 2023 at 08:48:59AM +0100, Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@intel.com>
> 
> Make sure that xdp_do_flush() is always executed before
> napi_complete_done(). This is important for two reasons. First, a
> redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
> napi context X on CPU Y will be followed by a xdp_do_flush() from the
> same napi context and CPU. This is not guaranteed if the
> napi_complete_done() is executed before xdp_do_flush(), as it tells
> the napi logic that it is fine to schedule napi context X on another
> CPU. Details from a production system triggering this bug using the
> veth driver can be found following the first link below.
> 
> The second reason is that the XDP_REDIRECT logic in itself relies on
> being inside a single NAPI instance through to the xdp_do_flush() call
> for RCU protection of all in-kernel data structures. Details can be
> found in the second link below.
> 
> Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
> Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
> Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
> Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 18b3de854aeb..6df14dd5bf46 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1677,13 +1677,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  
>  	received = virtnet_receive(rq, budget, &xdp_xmit);
>  
> +	if (xdp_xmit & VIRTIO_XDP_REDIR)
> +		xdp_do_flush();
> +
>  	/* Out of packets? */
>  	if (received < budget)
>  		virtqueue_napi_complete(napi, rq->vq, received);
>  
> -	if (xdp_xmit & VIRTIO_XDP_REDIR)
> -		xdp_do_flush();
> -
>  	if (xdp_xmit & VIRTIO_XDP_TX) {
>  		sq = virtnet_xdp_get_sq(vi);
>  		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> -- 
> 2.34.1
diff mbox series

Patch

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 18b3de854aeb..6df14dd5bf46 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1677,13 +1677,13 @@  static int virtnet_poll(struct napi_struct *napi, int budget)
 
 	received = virtnet_receive(rq, budget, &xdp_xmit);
 
+	if (xdp_xmit & VIRTIO_XDP_REDIR)
+		xdp_do_flush();
+
 	/* Out of packets? */
 	if (received < budget)
 		virtqueue_napi_complete(napi, rq->vq, received);
 
-	if (xdp_xmit & VIRTIO_XDP_REDIR)
-		xdp_do_flush();
-
 	if (xdp_xmit & VIRTIO_XDP_TX) {
 		sq = virtnet_xdp_get_sq(vi);
 		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {