On Thu, 29 Feb 2024 11:38:53 +1100
David Gibson
On Wed, Feb 28, 2024 at 02:26:18PM +0100, Laurent Vivier wrote:
On 2/19/24 04:08, David Gibson wrote:
On Sat, Feb 17, 2024 at 04:07:23PM +0100, Laurent Vivier wrote:
[...]
+/** + * proto_ipv6_header_psum() - Calculates the partial checksum of an + * IPv6 header for UDP or TCP + * @payload_len: Payload length + * @proto: Protocol number + * @saddr: Source address + * @daddr: Destination address + * Returns: Partial checksum of the IPv6 header + */ +uint32_t proto_ipv6_header_psum(uint16_t payload_len, uint8_t protocol, + struct in6_addr saddr, struct in6_addr daddr)
Hrm, this is passing 2 16-byte IPv6 addresses by value, which might not be what we want.
The idea here is to avoid the pointer alignment problem (&ip6h->saddr and &ip6h->daddr can be misaligned).
Ah, right. That's a neat idea, but I'm not sure it really helps: I think it will just move the misaligned access from inside the function to the call site, where we try to marshal the parameter from something unaligned.
I haven't tested this yet, but note that this is generally okay: the problem is *dereferencing* an unaligned pointer. But if you load memory from an aligned pointer, and extract a value from this memory, it's all fine. Speaking MIPS, this is not safe on all CPU models: la $1, 1002 # s1 now contains the value 1002 lw $2, 0($1) # load word from memory at 1002 + 0 into s2 but this is: la $1, 1000 # s1 now contains the value 1000 la $2, 1004 # s3 now contains the value 1004 lw $3, 0($1) # load word from memory at 1000 + 0 into s3 lw $4, 0($3) # load word from memory at 1004 + 0 into s4 sll $5, $3, 16 # 16-bit shift left s3 into s5 srl $6, $4, 16 # 16-bit shift right s4 into s6 or $2, $5, $6 # OR s5 and s6 into s2 On x86, as far as I know, mov will digest the equivalent of the first version without issues.
Is it a better solution to copy the content of ip6h->saddr and ip6h->daddr to some local variables and then provide the pointers of the local variables to proto_ipv6_header_psum()?
Honestly, I'm not sure.
I think it's pretty much the same. Let the compiler pass 16-byte variables by value, and it will generally do this for us, but only if needed. -- Stefano