linux-kernel - Re: [PATCH] powerpc/bpf: Only update ldimm64 during extra pass when it is an address

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Message-Id: <1669354227.c0iu04jq2o.naveen@linux.ibm.com>
Date:   Fri, 25 Nov 2022 11:08:51 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Hao Luo <haoluo@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Stanislav Fomichev <sdf@...gle.com>,
        Song Liu <song@...nel.org>,
        Yonghong
 Song <yhs@...com>
Subject: Re: [PATCH] powerpc/bpf: Only update ldimm64 during extra pass when
 it is an address

Christophe Leroy wrote:
> 
> 
> Le 24/11/2022 à 14:49, Naveen N. Rao a écrit :
>> Christophe Leroy wrote:
>>>
>>>
>>> Le 24/11/2022 à 11:13, Naveen N. Rao a écrit :
>>>> Christophe Leroy wrote:
>>>
>>> In what direction could that change in the future ?
>>>
>>> For me if they change that it becomes an API change.
>> 
>> More of an extension, which is exactly what we had when BPF_PSEUDO_FUNC 
>> was introduced. Took us nearly a year before we noticed.
>> 
>> Because we do not do a full JIT during the extra pass today like other 
>> architectures, we are the exception - there is always the risk of bpf 
>> core changes breaking our JIT. So, I still think it is better if we do a 
>> full JIT during extra pass.
>> 
> 
> I like the idea of a full JIT during extra passes and will start looking 
> at it.
> 
> Will it also allow us to revert your commit fab07611fb2e 
> ("powerpc32/bpf: Fix codegen for bpf-to-bpf calls") ?

Not entirely. We still need those extra nops during the initial JIT so 
that we can estimate the maximum prog size. During extra pass, we can 
only emit the necessary instructions and skip extra nops. We may need to 
do two passes during extra_pass to adjust the branch targets though.

Thanks,
Naveen