Eric Dumazet 91ff28ae6d x86/irqflags: Use ASM_OUTPUT_RM in native_save_fl()
clang is generating very inefficient code for native_save_fl() which is
used for local_irq_save() in critical spots.

Allowing the "pop %0" to use memory:

 1) forces the compiler to add annoying stack canaries when
    CONFIG_STACKPROTECTOR_STRONG=y in many places.

 2) Almost always is followed by an immediate "move memory,register"

One good example is _raw_spin_lock_irqsave, with 8 extra instructions

  ffffffff82067a30 <_raw_spin_lock_irqsave>:
  ffffffff82067a30:		...
  ffffffff82067a39:		53						push   %rbx

  // Three instructions to ajust the stack, read the per-cpu canary
  // and copy it to 8(%rsp)
  ffffffff82067a3a:		48 83 ec 10 			sub    $0x10,%rsp
  ffffffff82067a3e:		65 48 8b 05 da 15 45 02 mov    %gs:0x24515da(%rip),%rax 	   # <__stack_chk_guard>
  ffffffff82067a46:		48 89 44 24 08			mov    %rax,0x8(%rsp)

  ffffffff82067a4b:		9c						pushf

  // instead of pop %rbx, compiler uses 2 instructions.
  ffffffff82067a4c:		8f 04 24				pop    (%rsp)
  ffffffff82067a4f:		48 8b 1c 24 			mov    (%rsp),%rbx

  ffffffff82067a53:		fa						cli
  ffffffff82067a54:		b9 01 00 00 00			mov    $0x1,%ecx
  ffffffff82067a59:		31 c0					xor    %eax,%eax
  ffffffff82067a5b:		f0 0f b1 0f 			lock cmpxchg %ecx,(%rdi)
  ffffffff82067a5f:		75 1d					jne    ffffffff82067a7e <_raw_spin_lock_irqsave+0x4e>

  // three instructions to check the stack canary
  ffffffff82067a61:		65 48 8b 05 b7 15 45 02 mov    %gs:0x24515b7(%rip),%rax 	   # <__stack_chk_guard>
  ffffffff82067a69:		48 3b 44 24 08			cmp    0x8(%rsp),%rax
  ffffffff82067a6e:		75 17					jne    ffffffff82067a87

  ...

  // One extra instruction to adjust the stack.
  ffffffff82067a73:		48 83 c4 10 			add    $0x10,%rsp
  ...

  // One more instruction in case the stack was mangled.
  ffffffff82067a87:		e8 a4 35 ff ff			call   ffffffff8205b030 <__stack_chk_fail>

This patch changes nothing for gcc, but for clang saves ~20000 bytes of text
even though more functions are inlined.

  $ size vmlinux.gcc.before vmlinux.gcc.after vmlinux.clang.before vmlinux.clang.after
     text	   data		bss		dec		hex	filename
  45565821	25005462	4704800	75276083	47c9f33	vmlinux.gcc.before
  45565821	25005462	4704800	75276083	47c9f33	vmlinux.gcc.after
  45121072	24638617	5533040	75292729	47ce039	vmlinux.clang.before
  45093887	24638633	5536808	75269328	47c84d0	vmlinux.clang.after

  $ scripts/bloat-o-meter -t vmlinux.clang.before vmlinux.clang.after
  add/remove: 1/2 grow/shrink: 21/533 up/down: 2250/-22112 (-19862)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2025-12-20 14:47:12 -08:00
2025-02-19 14:53:27 -07:00
2025-12-14 16:05:07 +12:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst



Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst
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