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

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Message-Id: <1669297066.kxu8xl391n.naveen@linux.ibm.com>
Date:   Thu, 24 Nov 2022 19:19:29 +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 11:13, Naveen N. Rao a écrit :
>> Christophe Leroy wrote:
>>> ldimm64 is not only used for loading function addresses, and
>> 
>> That's probably true today, but I worry that that can change upstream 
>> and we may not notice at all.
> 
> Not sure what you mean.
> 
> Today POWERPC considers that ldimm64 is _always_ loading a function 
> address whereas upstream BPF considers that ldimm64 is a function only 
> when it is flagged BPF_PSEUDO_FUNC.

Not sure why you think we consider ldimm64 to always be loading a 
function address. Perhaps it is due to the poorly chosen variable name 
func_addr in bpf_jit_fixup_addresses(), or due to the fact that we 
always update the JIT code for ldimm64. In any case, we simply overwrite 
imm64 load instructions to ensure we are using the updated address.

> 
> 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.


- Naveen