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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Jan 2026 09:32:23 -0500

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

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

> > 
> > > 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,
> Stefano