Re: [PATCH v5 4/5] tcp: Update TCP checksum using an iovec array

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 27 Sep 2024 15:53:48 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> [...]
>
> +void tcp_update_check_tcp4(const struct iphdr *iph,
> +			   const struct iovec *iov, int iov_cnt,
> +			   size_t l4offset)
>  {
>  	uint16_t l4len = ntohs(iph->tot_len) - sizeof(struct iphdr);
>  	struct in_addr saddr = { .s_addr = iph->saddr };
>  	struct in_addr daddr = { .s_addr = iph->daddr };
> -	uint32_t sum = proto_ipv4_header_psum(l4len, IPPROTO_TCP, saddr, daddr);
> +	size_t check_ofs;
> +	__sum16 *check;
> +	int check_idx;
> +	uint32_t sum;
> +	uintptr_t ptr;
> +
> +	sum = proto_ipv4_header_psum(l4len, IPPROTO_TCP, saddr, daddr);
> +
> +	check_idx = iov_skip_bytes(iov, iov_cnt,
> +				   l4offset + offsetof(struct tcphdr, check),
> +				   &check_ofs);
> +
> +	if (check_idx >= iov_cnt) {
> +		err("TCP4 buffer is too small, iov size %zd, check offset %zd",
> +		    iov_size(iov, iov_cnt),
> +		    l4offset + offsetof(struct tcphdr, check));
> +		return;
> +	}
>  
> -	bp->th.check = 0;
> -	bp->th.check = csum(bp, l4len, sum);
> +	if (check_ofs + sizeof(*check) > iov[check_idx].iov_len) {
> +		err("TCP4 checksum field memory is not contiguous "
> +		    "check_ofs %zd check_idx %d iov_len %zd",
> +		    check_ofs, check_idx, iov[check_idx].iov_len);
> +		return;
> +	}
> +
> +	ptr = (uintptr_t)((char *)iov[check_idx].iov_base + check_ofs);
> +	if (ptr & (__alignof__(*check) - 1)) {
> +		err("TCP4 checksum field is not correctly aligned in memory");
> +		return;
> +	}
> +
> +	check = (__sum16 *)ptr;

clang-tidy (14.0.6) says:

/home/sbrivio/passt/tcp.c:803:10: error: integer to pointer cast pessimizes optimization opportunities [performance-no-int-to-ptr,-warnings-as-errors]
        check = (__sum16 *)ptr;
                ^
/home/sbrivio/passt/tcp.c:854:10: error: integer to pointer cast pessimizes optimization opportunities [performance-no-int-to-ptr,-warnings-as-errors]
        check = (__sum16 *)ptr;
                ^

...I'm still trying to figure out what that actually means. If it's
trivial I can fix that up on merge.

-- 
Stefano