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

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Message-ID: <aV-6gniRnZlNvkwc@sgarzare-redhat>
Date: Thu, 8 Jan 2026 15:11:36 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>, 
	Jonathan Corbet <corbet@....net>, Olivia Mackall <olivia@...enic.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, Jason Wang <jasowang@...hat.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>, 
	"Martin K. Petersen" <martin.petersen@...cle.com>, Gerd Hoffmann <kraxel@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Robin Murphy <robin.murphy@....com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Petr Tesarik <ptesarik@...e.com>, Leon Romanovsky <leon@...nel.org>, 
	Jason Gunthorpe <jgg@...pe.ca>, Bartosz Golaszewski <brgl@...nel.org>, linux-doc@...r.kernel.org, 
	linux-crypto@...r.kernel.org, virtualization@...ts.linux.dev, linux-scsi@...r.kernel.org, 
	iommu@...ts.linux.dev, kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 13/15] vsock/virtio: reorder fields to reduce padding

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@...hat.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
>