> -----Original Message-----
> From: Stephen Hemminger <step...@networkplumber.org>
> Sent: den 30 juni 2022 19:46
> To: Morten Brørup <m...@smartsharesystems.com>
> Cc: Emil Berg <emil.b...@ericsson.com>; bruce.richard...@intel.com;
> dev@dpdk.org; sta...@dpdk.org; bugzi...@dpdk.org; hof...@lysator.liu.se;
> olivier.m...@6wind.com
> Subject: Re: [PATCH v4] net: fix checksum with unaligned buffer
>
> On Thu, 23 Jun 2022 14:39:00 +0200
> Morten Brørup <m...@smartsharesystems.com> wrote:
>
> > + /* if buffer is unaligned, keeping it byte order independent */
> > + if (unlikely(unaligned)) {
> > + uint16_t first = 0;
> > + if (unlikely(len == 0))
> > + return 0;
>
> Why is length == 0 unique to unaligned case?
>
> > + ((unsigned char *)&first)[1] = *(const unsigned
> char *)buf;
>
> Use a proper union instead of casting to avoid aliasing warnings.
>
> > + bsum += first;
> > + buf = RTE_PTR_ADD(buf, 1);
> > + len--;
> > + }
>
> Many CPU's (such as x86) won't care about alignment and therefore the
> extra code to handle this is not worth doing.
>
x86 does care about alignment. An example is the vmovdqa instruction, where 'a'
stands for 'aligned'. The description in the link below says: "When the source
or destination operand is a memory operand, the operand must be aligned on a
16-byte boundary or a general-protection exception (#GP) will be generated. "
https://www.felixcloutier.com/x86/movdqa:vmovdqa32:vmovdqa64
> Perhaps DPDK needs a macro (like Linux kernel) for efficient unaligned
> access.
>
> In Linux kernel it is CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
