Re: [PATCH] parisc: fix a possible DMA corruption

[John David Anglin <dave.anglin@bell.net>]

On 2024-07-27 6:06 a.m., Mikulas Patocka wrote:
> ARCH_DMA_MINALIGN was defined as 16 - this is too small - it may be
> possible that two unrelated 16-byte allocations share a cache line. If one
> of these allocations is written using DMA and the other is written using
> cached write, the value that was written with DMA may be corrupted.
Agreed.
>
> This commit changes ARCH_DMA_MINALIGN to be 128 - that's the largest
> possible cache line size on parisc.
We could use 32 if CONFIG_PA20 isn't defined.
>
> As different parisc microarchitectures have different cache line size, we
> define arch_slab_minalign(), cache_line_size() and
> dma_get_cache_alignment() so that the kernel may tune slab cache
> parameters dynamically, based on the detected cache line size.
>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Cc: stable@vger.kernel.org
>
> ---
>   arch/parisc/Kconfig             |    1 +
>   arch/parisc/include/asm/cache.h |    7 ++++++-
>   2 files changed, 7 insertions(+), 1 deletion(-)
>
> Index: linux-6.10/arch/parisc/include/asm/cache.h
> ===================================================================
> --- linux-6.10.orig/arch/parisc/include/asm/cache.h	2023-09-18 11:33:40.000000000 +0200
> +++ linux-6.10/arch/parisc/include/asm/cache.h	2024-07-26 20:27:06.000000000 +0200
> @@ -20,7 +20,12 @@
>   
>   #define SMP_CACHE_BYTES L1_CACHE_BYTES
>   
> -#define ARCH_DMA_MINALIGN	L1_CACHE_BYTES
> +#define ARCH_DMA_MINALIGN	128
I would use 128 for if CONFIG_PA20 is defined and 32 otherwise.
> +#define ARCH_KMALLOC_MINALIGN	8
Looks okay if we don't need 16-byte alignment for ldcw.  Because PA 2.0 has
coherent ldcw support, we would only see this on PA 1.1 build.

Default is ARCH_DMA_MINALIGN.  So, current value is 16.
> +
> +#define arch_slab_minalign()	((unsigned)dcache_stride)
> +#define cache_line_size()	dcache_stride
> +#define dma_get_cache_alignment cache_line_size
>   
>   #define __read_mostly __section(".data..read_mostly")
>   
> Index: linux-6.10/arch/parisc/Kconfig
> ===================================================================
> --- linux-6.10.orig/arch/parisc/Kconfig	2024-07-23 20:35:34.000000000 +0200
> +++ linux-6.10/arch/parisc/Kconfig	2024-07-26 19:41:15.000000000 +0200
> @@ -20,6 +20,7 @@ config PARISC
>   	select ARCH_SUPPORTS_HUGETLBFS if PA20
>   	select ARCH_SUPPORTS_MEMORY_FAILURE
>   	select ARCH_STACKWALK
> +	select ARCH_HAS_CACHE_LINE_SIZE
>   	select ARCH_HAS_DEBUG_VM_PGTABLE
>   	select HAVE_RELIABLE_STACKTRACE
>   	select DMA_OPS
>

Dave

-- 
John David Anglin  dave.anglin@bell.net