Re: [PATCH bpf-next v1 3/3] selftests/bpf: tests for using dynptrs to parse skb and xdp buffers

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Joanne Koong <joannelkoong@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	bpf <bpf@vger.kernel.org>, Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>
Subject: Re: [PATCH bpf-next v1 3/3] selftests/bpf: tests for using dynptrs to parse skb and xdp buffers
Date: Thu, 4 Aug 2022 11:45:15 -0700	[thread overview]
Message-ID: <CAADnVQLEkfj-T8DXgmHU=MyUBL6Hb3TXPwNERzogW_DKCN2Asw@mail.gmail.com> (raw)
In-Reply-To: <CAJnrk1aodZ84YjaHNcxPZhREA+nx4=2Rh=4Nx9NcmkYvWn6S0g@mail.gmail.com>

On Wed, Aug 3, 2022 at 9:11 AM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > __builtin_memcpy() is best. When we write just "memcpy()" we still
> > rely on compiler to actually optimizing that to __builtin_memcpy(),
> > because there is no memcpy() (we'd get unrecognized extern error if
> > compiler actually emitted call to memcpy()).
>
> Ohh I see, thanks for the explanation!
>
> I am going to do some selftests cleanup this week, so I'll change the
> other usages of memcpy() to __builtin_memcpy() as part of that clean
> up.

builtin_memcpy might be doing single byte copy when
alignment is not known which is often the case when
working with packets.
If we do this cleanup, let's copy-paste cilium's memcpy
helper that does 8 byte copy.
It's much better than builtin_memcpy.
https://github.com/cilium/cilium/blob/master/bpf/include/bpf/builtins.h