Subject: [PATCH] arm64: cache: Lower ARCH_DMA_MINALIGN to 64 (L1_CACHE_BYTES)

From: Will Deacon <[email-masked]>
Date: Thu, 27 May 2021 13:43:56 +0100

Back in 97303480753e ("arm64: Increase the max granular size"),
ARCH_DMA_MINALIGN was effectively increased to 128 bytes thanks to an
increase in L1_CACHE_BYTES due to an unsubstantiated performance claim
on the now obsolete ThunderX-1. Although this was reverted in
d93277b9839b, ARCH_DMA_MINALIGN was kept at 128 bytes by ebc7e21e0fa2
("arm64: Increase ARCH_DMA_MINALIGN to 128").

During discussion of the original patch, it was reported that the change
also prevented a warning during boot on (again, now obsolete) Qualcomm
server hardware where the cache writeback granule was larger than 64
bytes. The reason for this warning was because non-coherent DMA could
lead to data corruption due to unexpected writeback from the CPU where a
cacheline is shared with other allocations.

Since then, systems have appeared with larger cachelines still, and so
commit 8f5c9037a55b ("arm64/mm: Correct the cache line size warning with
non coherent device") reworked the warning so that it only appears on
systems where non-coherent DMA is actually required and taints the
kernel with TAINT_CPU_OUT_OF_SPEC. We are not aware of any systems, even
including the aforementioned obsolete machines, which have a CWG larger
than 64 bytes and require non-coherent DMA.

More recently, it has been reported that a ARCH_DMA_MINALIGN of 128
bytes wastes considerable memory (~6% immediately after boot on one
system).

Reduce ARCH_DMA_MINALIGN to 64 bytes and allow the warning/taint to
indicate if there are machines that unknowingly rely on this.

Cc: Catalin Marinas <[email-masked]>
Cc: Mark Rutland <[email-masked]>
Cc: Ard Biesheuvel <[email-masked]>
Cc: Arnd Bergmann <[email-masked]>
Cc: Vincent Whitchurch <[email-masked]>
Link: https://lore.kernel.org/linux-arm-kernel/[email-masked]/
Link: https://lore.kernel.org/linux-arm-kernel/CAOZdJXUiRMAguDV+HEJqPg57MyBNqEcTyaH+ya=[email-masked]/
Link: https://lore.kernel.org/linux-arm-kernel/[email-masked]/
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Will Deacon <[email-masked]>
---
 arch/arm64/include/asm/cache.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
index a074459f8f2f..a9c0716e7440 100644
--- a/arch/arm64/include/asm/cache.h
+++ b/arch/arm64/include/asm/cache.h
@@ -47,7 +47,7 @@
  * cache before the transfer is done, causing old data to be seen by
  * the CPU.
  */
-#define ARCH_DMA_MINALIGN	(128)
+#define ARCH_DMA_MINALIGN	L1_CACHE_BYTES
 
 #ifdef CONFIG_KASAN_SW_TAGS
 #define ARCH_SLAB_MINALIGN	(1ULL << KASAN_SHADOW_SCALE_SHIFT)
-- 
2.31.1.818.g46aad6cb9e-goog

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