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

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Message-ID: <4dd462c8-7f0f-58c8-0f3f-757442134c90@csgroup.eu>
Date:   Fri, 25 Nov 2022 05:59:02 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        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



Le 25/11/2022 à 06:38, Naveen N. Rao a écrit :
> 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.
> 

Before your change, the code was:

	if (image && rel < 0x2000000 && rel >= -0x2000000) {
		PPC_BL(func);
	} else {
		/* Load function address into r0 */
		EMIT(PPC_RAW_LIS(_R0, IMM_H(func)));
		EMIT(PPC_RAW_ORI(_R0, _R0, IMM_L(func)));
		EMIT(PPC_RAW_MTCTR(_R0));
		EMIT(PPC_RAW_BCTRL());
	}

During the initial pass, image is NULL so the else branch is taken.

Christophe