arm64: mm: define NET_IP_ALIGN to 0

mainline inclusion
from mainline-v4.20-rc2
commit 26a4676faa1ad5d99317e0cd701e5d6f3e716b77
category: feature
bugzilla: NA
CVE: NA
---------------------------

On arm64, there is no need to add 2 bytes of padding to the start of
each network buffer just to make the IP header appear 32-bit aligned.

Since this might actually adversely affect DMA performance some
platforms, let's override NET_IP_ALIGN to 0 to get rid of this
padding.
Acked-by: NIlias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: NIlias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NHanjun Guo <guohanjun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

arch/arm64/include/asm/processor.h

@@ -24,6 +24,14 @@
 #define KERNEL_DS	UL(-1)
 #define USER_DS		(TASK_SIZE_64 - 1)
 
+/*
+ * On arm64 systems, unaligned accesses by the CPU are cheap, and so there is
+ * no point in shifting all network buffers by 2 bytes just to make some IP
+ * header fields appear aligned in memory, potentially sacrificing some DMA
+ * performance on some platforms.
+ */
+#define NET_IP_ALIGN	0
+
 #ifndef __ASSEMBLY__
 
 /*