[v2] fix DMA aligment issues around virtio

[PATCH v2 15/15] gpio: virtio: reorder fields to reduce struct padding

Posted by Michael S. Tsirkin 1 month ago

Reorder struct virtio_gpio_line fields to place the DMA buffers
(req/res) last.

This eliminates the padding from aligning struct size on
ARCH_DMA_MINALIGN.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/gpio/gpio-virtio.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
index b70294626770..ed6e0e90fa8a 100644
--- a/drivers/gpio/gpio-virtio.c
+++ b/drivers/gpio/gpio-virtio.c
@@ -26,11 +26,12 @@ struct virtio_gpio_line {
 	struct mutex lock; /* Protects line operation */
 	struct completion completion;
 
+	unsigned int rxlen;
+
 	__dma_from_device_group_begin();
 	struct virtio_gpio_request req;
 	struct virtio_gpio_response res;
 	__dma_from_device_group_end();
-	unsigned int rxlen;
 };
 
 struct vgpio_irq_line {
-- 
MST

Re: [PATCH v2 15/15] gpio: virtio: reorder fields to reduce struct padding

Posted by Bartosz Golaszewski 1 month ago

On Mon, 5 Jan 2026 09:23:49 +0100, "Michael S. Tsirkin" <mst@redhat.com> said:
> Reorder struct virtio_gpio_line fields to place the DMA buffers
> (req/res) last.
>
> This eliminates the padding from aligning struct size on
> ARCH_DMA_MINALIGN.
>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/gpio/gpio-virtio.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
> index b70294626770..ed6e0e90fa8a 100644
> --- a/drivers/gpio/gpio-virtio.c
> +++ b/drivers/gpio/gpio-virtio.c
> @@ -26,11 +26,12 @@ struct virtio_gpio_line {
>  	struct mutex lock; /* Protects line operation */
>  	struct completion completion;
>
> +	unsigned int rxlen;
> +
>  	__dma_from_device_group_begin();
>  	struct virtio_gpio_request req;
>  	struct virtio_gpio_response res;
>  	__dma_from_device_group_end();
> -	unsigned int rxlen;
>  };
>
>  struct vgpio_irq_line {
> --
> MST
>
>

Acked-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

[PATCH v3 15/15] vsock/virtio: reorder fields to reduce padding

Posted by Michael S. Tsirkin 1 week, 3 days ago

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.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

changes from v2:
	move event_lock and event_run too, to keep
	event things logically together, as suggested by
	Stefano Garzarella.

Note: this is the only change in v3 and it's cosmetic, so I am
not reposting the whole patchset.


 net/vmw_vsock/virtio_transport.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 999a0839726a..b333a7591b26 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -55,15 +55,6 @@ struct virtio_vsock {
 	int rx_buf_nr;
 	int rx_buf_max_nr;
 
-	/* 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();
-
 	u32 guest_cid;
 	bool seqpacket_allow;
 
@@ -77,6 +68,15 @@ 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)
-- 
MST

Re: [PATCH v3 15/15] vsock/virtio: reorder fields to reduce padding

Posted by Stefano Garzarella 1 week, 2 days ago

On Wed, Jan 28, 2026 at 03:31:21PM -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.
>
>Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>---
>
>changes from v2:
>	move event_lock and event_run too, to keep
>	event things logically together, as suggested by
>	Stefano Garzarella.

Thanks for that!

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>
>Note: this is the only change in v3 and it's cosmetic, so I am
>not reposting the whole patchset.
>
>
> net/vmw_vsock/virtio_transport.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>index 999a0839726a..b333a7591b26 100644
>--- a/net/vmw_vsock/virtio_transport.c
>+++ b/net/vmw_vsock/virtio_transport.c
>@@ -55,15 +55,6 @@ struct virtio_vsock {
> 	int rx_buf_nr;
> 	int rx_buf_max_nr;
>
>-	/* 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();
>-
> 	u32 guest_cid;
> 	bool seqpacket_allow;
>
>@@ -77,6 +68,15 @@ 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)
>-- 
>MST
>