netdev - Re: [PATCH v2 4/5] bpf ppc32: add support for BPF

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Message-ID: <6a991140-f0f8-dbfc-d596-ce1473251034@linux.ibm.com>
Date:   Tue, 14 Jun 2022 00:30:36 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Cc:     Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Jordan Niethe <jniethe5@...il.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH v2 4/5] bpf ppc32: add support for BPF_ATOMIC bitwise
 operations



On 11/06/22 10:44 pm, Christophe Leroy wrote:
> 
> 
> Le 10/06/2022 à 17:55, Hari Bathini a écrit :
>> Adding instructions for ppc32 for
>>
>> atomic_and
>> atomic_or
>> atomic_xor
>> atomic_fetch_add
>> atomic_fetch_and
>> atomic_fetch_or
>> atomic_fetch_xor
>>
>> Signed-off-by: Hari Bathini <hbathini@...ux.ibm.com>
>> ---
>>
>> Changes in v2:
>> * Used an additional register (BPF_REG_AX)
>>       - to avoid clobbering src_reg.
>>       - to keep the lwarx reservation as intended.
>>       - to avoid the odd switch/goto construct.
> 
> Might be a stupid question as I don't know the internals of BPF: Are we
> sure BPF_REG_AX cannot be the src reg or the dst reg ?
> 

AFAICS, BPF_REG_AX wouldn't be used as src_reg or dst_reg unless this
code is reused internally, by arch-specific code, for JIT'ing some other
instruction(s) using BPF_REG_AX as either src or dst reg..

Thanks
Hari