netdev - Re: [PATCH bpf-next 1/4] bpf: Add BPF_F_ANY

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Message-ID: <20181201055020.ebyugsxvroortswq@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 30 Nov 2018 21:50:22 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] bpf: Add BPF_F_ANY_ALIGNMENT.

On Fri, Nov 30, 2018 at 08:44:20PM -0800, David Miller wrote:
> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Date: Fri, 30 Nov 2018 13:58:20 -0800
> 
> > On Thu, Nov 29, 2018 at 07:32:41PM -0800, David Miller wrote:
> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >> index 426b5c8..c9647ea 100644
> >> --- a/include/uapi/linux/bpf.h
> >> +++ b/include/uapi/linux/bpf.h
> >> @@ -232,6 +232,16 @@ enum bpf_attach_type {
> >>   */
> >>  #define BPF_F_STRICT_ALIGNMENT	(1U << 0)
> >>  
> >> +/* If BPF_F_ANY_ALIGNMENT is used in BPF_PROF_LOAD command, the
> >> + * verifier will allow any alignment whatsoever.  This bypasses
> >> + * what CONFIG_EFFICIENT_UNALIGNED_ACCESS would cause it to do.
> > 
> > I think majority of user space folks who read uapi/bpf.h have no idea
> > what that kernel config does.
> > Could you reword the comment here to say that this flag is only
> > effective on architectures and like sparc and mips that don't
> > have efficient unaligned access and ignored on x86/arm64 ?
> 
> I just want to point out in passing that your feeback applies also to
> the comment above BPF_F_STRICT_ALIGNMENT, which I used as a model for
> my comment.

Good point. Missed that earlier.
NET_IP_ALIGN is even more cryptic and it's not the same as HAVE_EFFICIENT_UNALIGNED_ACCESS.
Example: s390
We need to reword it.