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

Stefano Garzarella
2026-01-08T14:27:25Z

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 <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.

>
>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