Re: [PATCH v2 13/15] vsock/virtio: reorder fields to reduce padding

[Stefano Garzarella]

On Thu, Jan 08, 2026 at 09:17:49AM -0500, Michael S. Tsirkin wrote:
On Thu, Jan 08, 2026 at 03:11:36PM +0100, Stefano Garzarella wrote:
On Mon, Jan 05, 2026 at 03:23:41AM -0500, Michael S. Tsirkin wrote:
> 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 <m...@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.


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


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)


Thanks,
Stefano