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

On Thu, May 27, 2021 at 01:43:56PM +0100, Will Deacon wrote:

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 <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Vincent Whitchurch <redacted>
Link: https://lore.kernel.org/linux-arm-kernel/1442944788-17254-1-git-send-email-rric@kernel.org/ (local)
Link: https://lore.kernel.org/linux-arm-kernel/CAOZdJXUiRMAguDV+HEJqPg57MyBNqEcTyaH+ya=U93NHb-pdJA@mail.gmail.com/ (local)
Link: https://lore.kernel.org/linux-arm-kernel/20190614131141.4428-1-msys.mizuma@gmail.com/ (local)
Link: https://lore.kernel.org/r/20210517074332.28280-1-vincent.whitchurch@axis.com (local)
Signed-off-by: Will Deacon <will@kernel.org>

Unless we hear from anyone that the warning is triggered, I think we
should change this in mainline.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

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