netdev - [PATCH v2 14/15] gpio: virtio: fix DMA alignment

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Message-ID: <ba7e025a6c84aed012421468d83639e5dae982b0.1767601130.git.mst@redhat.com>
Date: Mon, 5 Jan 2026 03:23:45 -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>,
	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,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Enrico Weigelt, metux IT consult" <info@...ux.net>,
	Viresh Kumar <vireshk@...nel.org>,
	Linus Walleij <linusw@...nel.org>, linux-gpio@...r.kernel.org
Subject: [PATCH v2 14/15] gpio: virtio: fix DMA alignment

The res and ires buffers in struct virtio_gpio_line and struct
vgpio_irq_line respectively are used for DMA_FROM_DEVICE via
virtqueue_add_sgs().  However, within these structs, even though these
elements are tagged as ____cacheline_aligned, adjacent struct elements
can share DMA cachelines on platforms where ARCH_DMA_MINALIGN >
L1_CACHE_BYTES (e.g., arm64 with 128-byte DMA alignment but 64-byte
cache lines).

The existing ____cacheline_aligned annotation aligns to L1_CACHE_BYTES
which is not always sufficient for DMA alignment. For example, with
L1_CACHE_BYTES = 32 and ARCH_DMA_MINALIGN = 128
  - irq_lines[0].ires at offset 128
  - irq_lines[1].type at offset 192
both in same 128-byte DMA cacheline [128-256)

When the device writes to irq_lines[0].ires and the CPU concurrently
modifies one of irq_lines[1].type/disabled/masked/queued flags,
corruption can occur on non-cache-coherent platforms.

Fix by using __dma_from_device_group_begin()/end() annotations on the
DMA buffers. Drop ____cacheline_aligned - it's not required to isolate
request and response, and keeping them would increase the memory cost.

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 drivers/gpio/gpio-virtio.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
index 17e040991e46..b70294626770 100644
--- a/drivers/gpio/gpio-virtio.c
+++ b/drivers/gpio/gpio-virtio.c
@@ -10,6 +10,7 @@
  */
 
 #include <linux/completion.h>
+#include <linux/dma-mapping.h>
 #include <linux/err.h>
 #include <linux/gpio/driver.h>
 #include <linux/io.h>
@@ -24,8 +25,11 @@
 struct virtio_gpio_line {
 	struct mutex lock; /* Protects line operation */
 	struct completion completion;
-	struct virtio_gpio_request req ____cacheline_aligned;
-	struct virtio_gpio_response res ____cacheline_aligned;
+
+	__dma_from_device_group_begin();
+	struct virtio_gpio_request req;
+	struct virtio_gpio_response res;
+	__dma_from_device_group_end();
 	unsigned int rxlen;
 };
 
@@ -37,8 +41,10 @@ struct vgpio_irq_line {
 	bool update_pending;
 	bool queue_pending;
 
-	struct virtio_gpio_irq_request ireq ____cacheline_aligned;
-	struct virtio_gpio_irq_response ires ____cacheline_aligned;
+	__dma_from_device_group_begin();
+	struct virtio_gpio_irq_request ireq;
+	struct virtio_gpio_irq_response ires;
+	__dma_from_device_group_end();
 };
 
 struct virtio_gpio {
-- 
MST