On Thu, Jan 08, 2026 at 09:32:23AM -0500, Michael S. Tsirkin wrote:

On Thu, Jan 08, 2026 at 03:27:04PM +0100, Stefano Garzarella wrote:

quoted

On Thu, Jan 08, 2026 at 09:17:49AM -0500, Michael S. Tsirkin wrote:

quoted

On Thu, Jan 08, 2026 at 03:11:36PM +0100, Stefano Garzarella wrote:

quoted

On Mon, Jan 05, 2026 at 03:23:41AM -0500, Michael S. Tsirkin wrote:

quoted

Reorder struct virtio_vsock fields to place the DMA buffer (event_list)
last. This eliminates the padding from aligning the struct size on
ARCH_DMA_MINALIGN.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
net/vmw_vsock/virtio_transport.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index ef983c36cb66..964d25e11858 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c

@@ -60,9 +60,7 @@ struct virtio_vsock {

	 */
	struct mutex event_lock;
	bool event_run;
-	__dma_from_device_group_begin();
-	struct virtio_vsock_event event_list[8];
-	__dma_from_device_group_end();
+
	u32 guest_cid;
	bool seqpacket_allow;

@@ -76,6 +74,10 @@ struct virtio_vsock {

	 */
	struct scatterlist *out_sgs[MAX_SKB_FRAGS + 1];
	struct scatterlist out_bufs[MAX_SKB_FRAGS + 1];
+

IIUC we would like to have these fields always on the bottom of this struct,
so would be better to add a comment here to make sure we will not add other
fields in the future after this?

not necessarily - you can add fields after, too - it's just that
__dma_from_device_group_begin already adds a bunch of padding, so adding
fields in this padding is cheaper.

Okay, I see.

quoted

do we really need to add comments to teach people about the art of
struct packing?

I can do it later if you prefer, I don't want to block this work, but yes,
I'd prefer to have a comment because otherwise I'll have to ask every time
to avoid, especially for new contributors xD

On the one hand you are right on the other I don't want it
duplicated each time __dma_from_device_group_begin is invoked.

yeah, I see.

Pls come up with something you like, and we'll discuss.

sure, I'll check a bit similar cases to have some inspiration.

quoted

quoted

quoted

Maybe we should also add a comment about the `ev`nt_lock`
requirement we
have in the section above.

Thanks,
Stefano

hmm which requirement do you mean?

That `event_list` must be accessed with `event_lock`.

So maybe we can move also `event_lock` and `event_run`, so we can just move
that comment. I mean something like this:

@@ -74,6 +67,15 @@ struct virtio_vsock {
         */
        struct scatterlist *out_sgs[MAX_SKB_FRAGS + 1];
        struct scatterlist out_bufs[MAX_SKB_FRAGS + 1];
+
+       /* The following fields are protected by event_lock.
+        * vqs[VSOCK_VQ_EVENT] must be accessed with event_lock held.
+        */
+       struct mutex event_lock;
+       bool event_run;
+       __dma_from_device_group_begin();
+       struct virtio_vsock_event event_list[8];
+       __dma_from_device_group_end();
 };

 static u32 virtio_transport_get_local_cid(void)

Yea this makes sense.

Thanks for that!
Stefano