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Paolo Bonzini 9e03b7caf4 KVM x86 misc changes for 6.20
- Disallow changing the virtual CPU model if L2 is active, for all the same
    reasons KVM disallows change the model after the first KVM_RUN.
 
  - Fix a bug where KVM would incorrectly reject host accesses to PV MSRs that
    were advertised as supported to userspace when running with
    KVM_CAP_ENFORCE_PV_FEATURE_CPUID enabled.
 
  - Fix a bug where KVM would attempt to read protect guest state (CR3) when
    configuring an async #PF entry.
 
  - Fail the build if EXPORT_SYMBOL_GPL or EXPORT_SYMBOL is used in KVM (for x86
    only) to enforce usage of EXPORT_SYMBOL_FOR_KVM_INTERNAL.  Explicitly allow
    the few exports that are intended for external usage.
 
  - Ignore -EBUSY when checking nested events after a vCPU exits blocking as
    the WARN is user-triggerable, and because exiting to userspace on -EBUSY
    does more harm than good in pretty much every situation.
 
  - Throw in the towel and drop the WARN on INIT/SIPI being blocked when vCPU is
    in Wait-For-SIPI, as playing whack-a-mole with syzkaller turned out to be an
    unwinnable game.
 
  - Add support for new Intel instructions that don't require anything beyond
    enumerating feature flags to userspace.
 
  - Grab SRCU when reading PDPTRs in KVM_GET_SREGS2.
 
  - Add WARNs to guard against modifying KVM's CPU caps outside of the intended
    setup flow, as nested VMX in particular is sensitive to unexpected changes
    in KVM's golden configuration.
 
  - Add a quirk to allow userspace to opt-in to actually suppress EOI broadcasts
    when the suppression feature is enabled by the guest (currently limited to
    split IRQCHIP, i.e. userspace I/O APIC).  Sadly, simply fixing KVM to honor
    Suppress EOI Broadcasts isn't an option as some userspaces have come to rely
    on KVM's buggy behavior (KVM advertises Supress EOI Broadcast irrespective
    of whether or not userspace I/O APIC supports Directed EOIs).
 
  - Minor cleanups.
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Merge tag 'kvm-x86-misc-6.20' of https://github.com/kvm-x86/linux into HEAD

KVM x86 misc changes for 6.20

 - Disallow changing the virtual CPU model if L2 is active, for all the same
   reasons KVM disallows change the model after the first KVM_RUN.

 - Fix a bug where KVM would incorrectly reject host accesses to PV MSRs that
   were advertised as supported to userspace when running with
   KVM_CAP_ENFORCE_PV_FEATURE_CPUID enabled.

 - Fix a bug where KVM would attempt to read protect guest state (CR3) when
   configuring an async #PF entry.

 - Fail the build if EXPORT_SYMBOL_GPL or EXPORT_SYMBOL is used in KVM (for x86
   only) to enforce usage of EXPORT_SYMBOL_FOR_KVM_INTERNAL.  Explicitly allow
   the few exports that are intended for external usage.

 - Ignore -EBUSY when checking nested events after a vCPU exits blocking as
   the WARN is user-triggerable, and because exiting to userspace on -EBUSY
   does more harm than good in pretty much every situation.

 - Throw in the towel and drop the WARN on INIT/SIPI being blocked when vCPU is
   in Wait-For-SIPI, as playing whack-a-mole with syzkaller turned out to be an
   unwinnable game.

 - Add support for new Intel instructions that don't require anything beyond
   enumerating feature flags to userspace.

 - Grab SRCU when reading PDPTRs in KVM_GET_SREGS2.

 - Add WARNs to guard against modifying KVM's CPU caps outside of the intended
   setup flow, as nested VMX in particular is sensitive to unexpected changes
   in KVM's golden configuration.

 - Add a quirk to allow userspace to opt-in to actually suppress EOI broadcasts
   when the suppression feature is enabled by the guest (currently limited to
   split IRQCHIP, i.e. userspace I/O APIC).  Sadly, simply fixing KVM to honor
   Suppress EOI Broadcasts isn't an option as some userspaces have come to rely
   on KVM's buggy behavior (KVM advertises Supress EOI Broadcast irrespective
   of whether or not userspace I/O APIC supports Directed EOIs).

 - Minor cleanups.
History 2026-02-09 18:53:47 +01:00
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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