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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Jan 2026 09:17:49 -0500

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 <[email-masked]>
> > ---
> > 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.

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

> Maybe we should also add a comment about the `event_lock` requirement we
> have in the section above.
> 
> Thanks,
> Stefano

hmm which requirement do you mean?

> 
> > +	__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)
> > -- 
> > MST
> >