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Sean Christopherson 6418b715d5 KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock
When creating a vCPU, don't drop kvm->lock to when doing the bulk of actual
vCPU creation, as allowing multiple vCPUs to be created in parallel adds
significant complexity in KVM (as evidenced by the many related bugs), and
all known VMMs fully serialize vCPU creation.  Remove all manually locking
of kvm->lock from kvm_arch_vcpu_{post,}create() for obvious reasons.

For many years, "everyone" has assumed that dropping kvm->lock was done for
performance reasons optimization, e.g. to allow userspace to create all
vCPUs concurrently for latency purposes.  But as above, no known VMM does
that.  Looking at the history of this code, before commit 11ec280471
("KVM: Convert vm lock to a mutex"), kvm->lock was a spinlock.  I.e. KVM
*had* to drop kvm->lock when doing the bulk of vCPU creation, otherwise KVM
couldn't do normal memory allocations.  When kvm->lock got turned into a
mutex for unrelated reasons, no one took advantage updated of the change to
simplify vCPU creation.  And 19 years later, everyone just assumed that KVM
continued to deal with the complexity for performance reasons.

Furthermore, naively parallelizing vCPU creation in userspace is likely a
net negative due to the overheads of task creation.  Unless a VMM carefully
avoids the extra overhead related to parallelization, e.g. spawns each
vCPU's thread before creating the vCPU, creating vCPUs concurrently is a
net *negative* up until about ~64 vCPUs, after which the times are a wash.

The absolute speed of light _is_ faster if KVM doesn't hold kvm-lock, but
at vCPU counts of ~16 or less, it's probably in the noise when considering
total VM creation time, as the added latency is less than 1ms up until 16
or so vCPUs.

On top of all that, KVM has had a *lot* of fatal bugs (most often found by
syzkaller) related to vCPUs being created while trying to do per-VM
operations (basically, see every flow that locks all vCPUs).  I.e. the
parallel vCPU creation "support" is actively harmful as the only "use case"
is for misbehaving userspace to exploit KVM bugs.

Serializing vCPU creation will allow reverting commit 97d65b544f ("KVM:
Check for duplicate vcpu_id as early as possible"), which had "minor" math
error: the worst case scenario isn't "256 bytes per VM", it's "256 unsigned
longs per VM", i.e. 2048 bytes per VM, which doubles the size of each VM
and pushes several architectures into order-1 allocations.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Jean-Christophe Guillain <jean-christophe@guillain.net>
Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Message-ID: <20260921174445.911676-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
History 2026-09-26 00:49:49 -04:00
arch KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock 2026-09-26 00:49:49 -04:00
block block: save page offset gaps in cloned bio 2026-09-01 08:42:34 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto This push fixes a memory allocation overrun in crypto acomp. 2026-08-28 09:28:40 -07:00
Documentation Changes since last update: 2026-09-12 08:18:50 -07:00
drivers KVM/arm64 changes for 7.3, take #2 2026-09-21 06:09:47 -04:00
fs xfs: fixes for 7.3-rc3 2026-09-12 08:44:12 -07:00
include KVM fixes for 7.3-rcN 2026-09-26 00:40:07 -04:00
init Bootconfig fixes for v7.3-rc3 2026-09-13 09:16:36 -07:00
io_uring io_uring-7.3-20260911 2026-09-11 12:36:13 -07:00
ipc mm.git review status for master..mm-nonmm-stable 2026-08-23 08:07:11 -07:00
kernel tracing fixes for 7.3: 2026-09-13 12:27:00 -07:00
lib block-7.3-20260911 2026-09-11 12:38:44 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm slab fixes for 7.3-rc2 2026-09-11 11:56:33 -07:00
net Nothing too exciting, usual stream of fixes. 2026-09-10 14:07:48 -07:00
rust Rust fixes for v7.3 (2nd) 2026-09-13 09:28:28 -07:00
samples rust: samples: add missing newlines in rust_print_main 2026-09-01 02:20:22 +02:00
scripts Fix misc objtool bugs: 2026-09-13 08:37:11 -07:00
security landlock: Test trace path output boundaries 2026-09-08 11:49:39 +02:00
sound sound fixes for 7.3-rc3 2026-09-11 10:00:07 -07:00
tools KVM fixes for 7.3-rcN 2026-09-26 00:40:07 -04:00
usr usr: Correct a spelling by changing a letter 2026-08-05 14:51:58 +02:00
virt KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock 2026-09-26 00:49:49 -04:00
.clang-format drm/bridge: rename drm_for_each_bridge_in_chain_scoped() to drm_for_each_bridge_in_chain() 2026-07-14 17:55:51 +02:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add Nathan Chancellor 2026-08-03 21:10:12 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap mailmap: update entry for Jens Axboe 2026-09-11 15:41:26 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS Driver core fixes for 7.3-rc2 2026-09-05 11:59:05 -07:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS IOMMU fixes for Linux v7.3-rc3: 2026-09-12 07:55:52 -07:00
Makefile Linux 7.3-rc3 2026-09-13 14:38:02 -07:00
README README: remove out of place emdashes 2026-07-15 10:51:29 -06:00

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
* AI Coding Assistant: LLMs and AI-powered development tools


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

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


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