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

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Message-Id: <20181130.204420.1837399220297298636.davem@davemloft.net>
Date:   Fri, 30 Nov 2018 20:44:20 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     alexei.starovoitov@...il.com
Cc:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] bpf: Add BPF_F_ANY_ALIGNMENT.

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.