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];
+
+	__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

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?

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

Thanks,
Stefano

>+	__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
>

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.


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
> >

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

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 <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

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

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:
>> 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
>
>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.

>
>> >
>> > > 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