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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
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