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For each tunable declaration, we create a zfs_tunable_t with its details, and then a pointer to it in the 'zfs_tunables' ELF section, that we can access later with a little support from the linker. Sponsored-by: https://despairlabs.com/sponsor/ Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Alexander Motin <mav@FreeBSD.org> Signed-off-by: Rob Norris <robn@despairlabs.com> Closes #17537
73 lines
2.9 KiB
C
73 lines
2.9 KiB
C
// SPDX-License-Identifier: CDDL-1.0
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/*
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* CDDL HEADER START
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*
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* The contents of this file are subject to the terms of the
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* Common Development and Distribution License (the "License").
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* You may not use this file except in compliance with the License.
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*
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* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
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* or https://opensource.org/licenses/CDDL-1.0.
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* See the License for the specific language governing permissions
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* and limitations under the License.
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*
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* When distributing Covered Code, include this CDDL HEADER in each
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* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
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* If applicable, add the following below this CDDL HEADER, with the
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* fields enclosed by brackets "[]" replaced with your own identifying
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* information: Portions Copyright [yyyy] [name of copyright owner]
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*
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* CDDL HEADER END
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*/
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/*
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* Copyright (c) 2025, Rob Norris <robn@despairlabs.com>
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*/
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#include <stddef.h>
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#include <sys/tunables.h>
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/*
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* Userspace tunables.
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*
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* Tunables are external pointers to global variables that are wired up to the
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* host environment in some way that allows the operator to directly change
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* their values "under the hood".
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*
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* In userspace, the "host environment" is the program using libzpool.so. So
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* that it can manipulate tunables if it wants, we provide an API to access
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* them.
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*
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* Tunables are declared through the ZFS_MODULE_PARAM* macros, which associate
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* a global variable with some metadata we can use to describe and access the
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* tunable. This is done by creating a uniquely-named zfs_tunable_t.
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*
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* At runtime, we need a way to discover these zfs_tunable_t items. Since they
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* are declared globally, all over the codebase, there's no central place to
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* record or list them. So, we take advantage of the compiler's "linker set"
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* feature.
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*
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* In the ZFS_MODULE_PARAM macro, after we create the zfs_tunable_t, we also
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* create a zfs_tunable_t* pointing to it. That pointer is forced into the
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* "zfs_tunables" ELF section in compiled object. At link time, the linker will
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* collect all these pointers into one single big "zfs_tunable" section, and
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* will generate two new symbols in the final object: __start_zfs_tunable and
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* __stop_zfs_tunable. These point to the first and last item in that section,
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* which allows us to access the pointers in that section like an array, and
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* through those pointers access the tunable metadata, and from there the
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* actual C variable that the tunable describes.
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*/
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extern const zfs_tunable_t *__start_zfs_tunables;
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extern const zfs_tunable_t *__stop_zfs_tunables;
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/*
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* Because there are no tunables in libspl itself, the above symbols will not
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* be generated, which will stop libspl being linked at all. To work around
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* that, we force a symbol into that section, and then when iterating, skip
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* any NULL pointers.
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*/
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static void *__zfs_tunable__placeholder
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__attribute__((__section__("zfs_tunables")))
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__attribute__((__used__)) = NULL;
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