[PATCH RFC 14/13] gpio: virtio: fix DMA alignment

[PATCH RFC 14/13] gpio: virtio: fix DMA alignment

[Michael S. Tsirkin]

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 now 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 platform.

Fix by using __dma_from_device_aligned_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.

Signed-off-by: Michael S. Tsirkin <mst@redhat.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..32b578b46df8 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 +25,12 @@
 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_aligned_begin
+	struct virtio_gpio_request req;
+	struct virtio_gpio_response res;
+
+	__dma_from_device_aligned_end
 	unsigned int rxlen;
 };
 
@@ -37,8 +42,9 @@ 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_aligned_begin
+	struct virtio_gpio_irq_request ireq;
+	struct virtio_gpio_irq_response ires;
 };
 
 struct virtio_gpio {
-- 
MST

[PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Michael S. Tsirkin]

Reorder struct virtio_gpio_line fields to place the DMA buffers (req/res)
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:
- request is 8 bytes
- response is 2 bytes
- removing _end saves up to 128-6=122 bytes padding to align rxlen field

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/gpio/gpio-virtio.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
index 32b578b46df8..8b30a94e4625 100644
--- a/drivers/gpio/gpio-virtio.c
+++ b/drivers/gpio/gpio-virtio.c
@@ -26,12 +26,11 @@ struct virtio_gpio_line {
 	struct mutex lock; /* Protects line operation */
 	struct completion completion;
 
+	unsigned int rxlen;
+
 	__dma_from_device_aligned_begin
 	struct virtio_gpio_request req;
 	struct virtio_gpio_response res;
-
-	__dma_from_device_aligned_end
-	unsigned int rxlen;
 };
 
 struct vgpio_irq_line {
-- 
MST

Re: [PATCH RFC 14/13] gpio: virtio: fix DMA alignment

[Bartosz Golaszewski]

On Tue, 30 Dec 2025 17:40:28 +0100, "Michael S. Tsirkin" <mst@redhat.com> said:
> 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 now 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 platform.
>
> Fix by using __dma_from_device_aligned_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.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---

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

Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Bartosz Golaszewski]

On Tue, 30 Dec 2025 17:40:33 +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 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:
> - request is 8 bytes
> - response is 2 bytes
> - removing _end saves up to 128-6=122 bytes padding to align rxlen field
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/gpio/gpio-virtio.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
> index 32b578b46df8..8b30a94e4625 100644
> --- a/drivers/gpio/gpio-virtio.c
> +++ b/drivers/gpio/gpio-virtio.c
> @@ -26,12 +26,11 @@ struct virtio_gpio_line {
>  	struct mutex lock; /* Protects line operation */
>  	struct completion completion;
>
> +	unsigned int rxlen;
> +
>  	__dma_from_device_aligned_begin
>  	struct virtio_gpio_request req;
>  	struct virtio_gpio_response res;
> -
> -	__dma_from_device_aligned_end
> -	unsigned int rxlen;
>  };
>
>  struct vgpio_irq_line {
> --
> MST
>
>

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

Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Michael S. Tsirkin]

On Fri, Jan 02, 2026 at 12:47:04PM +0000, Bartosz Golaszewski wrote:
> On Tue, 30 Dec 2025 17:40:33 +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 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:
> > - request is 8 bytes
> > - response is 2 bytes
> > - removing _end saves up to 128-6=122 bytes padding to align rxlen field
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  drivers/gpio/gpio-virtio.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
> > index 32b578b46df8..8b30a94e4625 100644
> > --- a/drivers/gpio/gpio-virtio.c
> > +++ b/drivers/gpio/gpio-virtio.c
> > @@ -26,12 +26,11 @@ struct virtio_gpio_line {
> >  	struct mutex lock; /* Protects line operation */
> >  	struct completion completion;
> >
> > +	unsigned int rxlen;
> > +
> >  	__dma_from_device_aligned_begin
> >  	struct virtio_gpio_request req;
> >  	struct virtio_gpio_response res;
> > -
> > -	__dma_from_device_aligned_end
> > -	unsigned int rxlen;
> >  };
> >
> >  struct vgpio_irq_line {
> > --
> > MST
> >
> >
> 
> Acked-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

Thanks! There's a new API as suggested by Petr so these patches got changed,
but the same idea. Do you want me to carry your ack or you prefer to
re-review?

-- 
MST

Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Bartosz Golaszewski]

On Fri, Jan 2, 2026 at 2:02 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Jan 02, 2026 at 12:47:04PM +0000, Bartosz Golaszewski wrote:
> > On Tue, 30 Dec 2025 17:40:33 +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 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:
> > > - request is 8 bytes
> > > - response is 2 bytes
> > > - removing _end saves up to 128-6=122 bytes padding to align rxlen field
> > >
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > >  drivers/gpio/gpio-virtio.c | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
> > > index 32b578b46df8..8b30a94e4625 100644
> > > --- a/drivers/gpio/gpio-virtio.c
> > > +++ b/drivers/gpio/gpio-virtio.c
> > > @@ -26,12 +26,11 @@ struct virtio_gpio_line {
> > >     struct mutex lock; /* Protects line operation */
> > >     struct completion completion;
> > >
> > > +   unsigned int rxlen;
> > > +
> > >     __dma_from_device_aligned_begin
> > >     struct virtio_gpio_request req;
> > >     struct virtio_gpio_response res;
> > > -
> > > -   __dma_from_device_aligned_end
> > > -   unsigned int rxlen;
> > >  };
> > >
> > >  struct vgpio_irq_line {
> > > --
> > > MST
> > >
> > >
> >
> > Acked-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
>
> Thanks! There's a new API as suggested by Petr so these patches got changed,
> but the same idea. Do you want me to carry your ack or you prefer to
> re-review?
>
> --
> MST
>

I'll take a second look. Can you Cc me on all the key patches - like
the ones introducing new APIs? I needed to grab it from lore this
time.

Bart

Re: [PATCH RFC 14/13] gpio: virtio: fix DMA alignment

[Viresh Kumar]

On 30-12-25, 11:40, Michael S. Tsirkin wrote:
> 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 now 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 platform.
> 
> Fix by using __dma_from_device_aligned_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.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/gpio/gpio-virtio.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 
viresh

Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Viresh Kumar]

On 30-12-25, 11:40, Michael S. Tsirkin wrote:
> Reorder struct virtio_gpio_line fields to place the DMA buffers (req/res)
> 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:
> - request is 8 bytes
> - response is 2 bytes
> - removing _end saves up to 128-6=122 bytes padding to align rxlen field
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/gpio/gpio-virtio.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 
viresh