linux-kernel - [PATCH RFC 13/13] vsock/virtio: reorder fields to reduce struct padding

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Message-ID: <dc7de7774ae19968549cf17336c15503fa7d10ec.1767089672.git.mst@redhat.com>
Date: Tue, 30 Dec 2025 05:16:29 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: 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>,
	Stefano Garzarella <sgarzare@...hat.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>,
	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: [PATCH RFC 13/13] vsock/virtio: reorder fields to reduce struct
 padding

Reorder struct virtio_vsock fields to place the DMA buffer (event_list)
last. This eliminates the need for __dma_from_device_aligned_end padding
after the DMA buffer, since struct tail padding naturally protects it,
making the struct a bit smaller.

Size reduction estimation when ARCH_DMA_MINALIGN=128:
- event_list is 32 bytes
- removing _end saves up to 128-32=96 bytes padding to align next field

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 net/vmw_vsock/virtio_transport.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index f1589db5d190..2e34581f1143 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -60,10 +60,7 @@ struct virtio_vsock {
 	 */
 	struct mutex event_lock;
 	bool event_run;
-	__dma_from_device_aligned_begin
-	struct virtio_vsock_event event_list[8];
 
-	__dma_from_device_aligned_end
 	u32 guest_cid;
 	bool seqpacket_allow;
 
@@ -77,6 +74,10 @@ struct virtio_vsock {
 	 */
 	struct scatterlist *out_sgs[MAX_SKB_FRAGS + 1];
 	struct scatterlist out_bufs[MAX_SKB_FRAGS + 1];
+
+	/* DMA buffer - must be last, aligned for non-cache-coherent DMA */
+	__dma_from_device_aligned_begin
+	struct virtio_vsock_event event_list[8];
 };
 
 static u32 virtio_transport_get_local_cid(void)
-- 
MST