From 0d74c42f788caf3cad727c61c490d9459bc8918b Mon Sep 17 00:00:00 2001 From: Joe Perches Date: Thu, 5 Dec 2013 14:54:38 -0800 Subject: [PATCH] ether_addr_equal: Optimize implementation, remove unused compare_ether_addr Add a new check for CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to reduce the number of or's used in the ether_addr_equal comparison to very slightly improve function performance. Simplify the ether_addr_equal_64bits implementation. Integrate and remove the zap_last_2bytes helper as it's now used only once. Remove the now unused compare_ether_addr function. Update the unaligned-memory-access documentation to remove the compare_ether_addr description and show how unaligned accesses could occur with ether_addr_equal. Signed-off-by: Joe Perches Signed-off-by: David S. Miller --- Documentation/unaligned-memory-access.txt | 28 +++++++++---- include/linux/etherdevice.h | 51 ++++++++--------------- 2 files changed, 37 insertions(+), 42 deletions(-) diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt index f866c72291bf..a445da098bc6 100644 --- a/Documentation/unaligned-memory-access.txt +++ b/Documentation/unaligned-memory-access.txt @@ -137,24 +137,34 @@ Code that causes unaligned access ================================= With the above in mind, let's move onto a real life example of a function -that can cause an unaligned memory access. The following function adapted +that can cause an unaligned memory access. The following function taken from include/linux/etherdevice.h is an optimized routine to compare two ethernet MAC addresses for equality. -unsigned int compare_ether_addr(const u8 *addr1, const u8 *addr2) +bool ether_addr_equal(const u8 *addr1, const u8 *addr2) { - const u16 *a = (const u16 *) addr1; - const u16 *b = (const u16 *) addr2; +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS + u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) | + ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4))); + + return fold == 0; +#else + const u16 *a = (const u16 *)addr1; + const u16 *b = (const u16 *)addr2; return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0; +#endif } -In the above function, the reference to a[0] causes 2 bytes (16 bits) to -be read from memory starting at address addr1. Think about what would happen -if addr1 was an odd address such as 0x10003. (Hint: it'd be an unaligned -access.) +In the above function, when the hardware has efficient unaligned access +capability, there is no issue with this code. But when the hardware isn't +able to access memory on arbitrary boundaries, the reference to a[0] causes +2 bytes (16 bits) to be read from memory starting at address addr1. + +Think about what would happen if addr1 was an odd address such as 0x10003. +(Hint: it'd be an unaligned access.) Despite the potential unaligned access problems with the above function, it -is included in the kernel anyway but is understood to only work on +is included in the kernel anyway but is understood to only work normally on 16-bit-aligned addresses. It is up to the caller to ensure this alignment or not use this function at all. This alignment-unsafe function is still useful as it is a decent optimization for the cases when you can ensure alignment, diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h index fc4a9aa7dd82..3526e819d7ae 100644 --- a/include/linux/etherdevice.h +++ b/include/linux/etherdevice.h @@ -26,6 +26,7 @@ #include #include #include +#include #ifdef __KERNEL__ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev); @@ -210,41 +211,27 @@ static inline void eth_hw_addr_inherit(struct net_device *dst, memcpy(dst->dev_addr, src->dev_addr, ETH_ALEN); } -/** - * compare_ether_addr - Compare two Ethernet addresses - * @addr1: Pointer to a six-byte array containing the Ethernet address - * @addr2: Pointer other six-byte array containing the Ethernet address - * - * Compare two Ethernet addresses, returns 0 if equal, non-zero otherwise. - * Unlike memcmp(), it doesn't return a value suitable for sorting. - */ -static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2) -{ - const u16 *a = (const u16 *) addr1; - const u16 *b = (const u16 *) addr2; - - BUILD_BUG_ON(ETH_ALEN != 6); - return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0; -} - /** * ether_addr_equal - Compare two Ethernet addresses * @addr1: Pointer to a six-byte array containing the Ethernet address * @addr2: Pointer other six-byte array containing the Ethernet address * * Compare two Ethernet addresses, returns true if equal + * + * Please note: addr1 & addr2 must both be aligned to u16. */ static inline bool ether_addr_equal(const u8 *addr1, const u8 *addr2) { - return !compare_ether_addr(addr1, addr2); -} +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) + u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) | + ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4))); -static inline unsigned long zap_last_2bytes(unsigned long value) -{ -#ifdef __BIG_ENDIAN - return value >> 16; + return fold == 0; #else - return value << 16; + const u16 *a = (const u16 *)addr1; + const u16 *b = (const u16 *)addr2; + + return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) == 0; #endif } @@ -265,16 +252,14 @@ static inline unsigned long zap_last_2bytes(unsigned long value) static inline bool ether_addr_equal_64bits(const u8 addr1[6+2], const u8 addr2[6+2]) { -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS - unsigned long fold = ((*(unsigned long *)addr1) ^ - (*(unsigned long *)addr2)); +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64 + u64 fold = (*(const u64 *)addr1) ^ (*(const u64 *)addr2); - if (sizeof(fold) == 8) - return zap_last_2bytes(fold) == 0; - - fold |= zap_last_2bytes((*(unsigned long *)(addr1 + 4)) ^ - (*(unsigned long *)(addr2 + 4))); - return fold == 0; +#ifdef __BIG_ENDIAN + return (fold >> 16) == 0; +#else + return (fold << 16) == 0; +#endif #else return ether_addr_equal(addr1, addr2); #endif