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([2620:10d:c090:500::5:99aa]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29601972a87sm39852035ad.15.2025.11.04.16.16.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Nov 2025 16:16:22 -0800 (PST) Message-ID: <3986a6b863be2ec62820ea5d2cf471f7e233fac0.camel@gmail.com> Subject: Re: [RFC PATCH v4 2/7] libbpf: Add BTF permutation support for type reordering From: Eduard Zingerman To: Andrii Nakryiko , Donglin Peng Cc: ast@kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Alan Maguire , Song Liu , pengdonglin Date: Tue, 04 Nov 2025 16:16:21 -0800 In-Reply-To: References: <20251104134033.344807-1-dolinux.peng@gmail.com> <20251104134033.344807-3-dolinux.peng@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-1.fc42) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-11-04 at 16:11 -0800, Andrii Nakryiko wrote: [...] > > +static int btf_permute_remap_type_id(__u32 *type_id, void *ctx) > > +{ > > + struct btf_permute *p =3D ctx; > > + __u32 new_type_id =3D *type_id; > > + > > + /* skip references that point into the base BTF */ > > + if (new_type_id < p->btf->start_id) > > + return 0; > > + > > + new_type_id =3D p->map[*type_id - p->btf->start_id]; >=20 > I'm actually confused, I thought p->ids would be the mapping from > original type ID (minus start_id, of course) to a new desired ID, but > it looks to be the other way? ids is a desired resulting *sequence* of > types identified by their original ID. I find it quite confusing. I > think about permutation as a mapping from original type ID to a new > type ID, am I confused? Yes, it is a desired sequence, not mapping. I guess its a bit simpler to use for sorting use-case, as you can just swap ids while sorting.