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

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Message-ID: <nyz6mnesozpu5u6p2mxrg37fwuj3sy7hjo2xkyepd3aybm7m52@7weoocg2pbs5>
Date: Mon, 5 Jan 2026 10:26:40 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
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>, 
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	"Enrico Weigelt, metux IT consult" <info@...ux.net>, Viresh Kumar <vireshk@...nel.org>, 
	Linus Walleij <linusw@...nel.org>, Bartosz Golaszewski <brgl@...nel.org>, 
	linux-gpio@...r.kernel.org
Subject: Re: [PATCH RFC 14/13] gpio: virtio: fix DMA alignment

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@...hat.com>
> ---
>  drivers/gpio/gpio-virtio.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)

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

-- 
viresh