[PATCH v4 00/10] KVM: Register cpuhp/syscore callbacks when enabling virt

[Sean Christopherson <seanjc@google.com>]

Register KVM's cpuhp and syscore callbacks when enabling virtualization in
hardware, as the sole purpose of said callbacks is to disable and re-enable
virtualization as needed.

The primary motivation for this series is to simplify dealing with enabling
virtualization for Intel's TDX, which needs to enable virtualization
when kvm-intel.ko is loaded, i.e. long before the first VM is created.

That said, this is a nice cleanup on its own.  By registering the callbacks
on-demand, the callbacks themselves don't need to check kvm_usage_count,
because their very existence implies a non-zero count.

Patch 1 (re)adds a dedicated lock for kvm_usage_count.  Hopefully it's a
temporary lock, as we can likely fix the cpus_read_lock() vs. kvm_lock mess
for good by switching vm_list to an (S)RCU-protected list.

v4:
 - Collect more reviews.
 - Fix documentation for kvm_usage_lock. [Kai]
 - Add a blurb in locking.rst to call out that cpus_read_lock() vs kvm_lock is
   a general problem.
 - Add a paragraph in the locking patch to suggest switching vm_list to an
   (S)RCU-protected list, so that walkers don't need to take kvm_lock.
 - Add an example call chain for the other cpus_read_lock() vs kvm_lock issues
   that are lurking.
 - Enable the module param by default from the get-go. [Paolo]
 - Rename even more APIs/symbols (all of 'em I could find). [Paolo]
 - Clarify that it's the 0=>1 VM creation that's problematic, not simply the
   creation of the very first VM. [Paolo]
 - Document enable_virt_at_load, and call out the (dis)advantages of enabling
   virtualization when KVM is loaded.
 - Drop the WARN on kvm_usage_count being elevated at the end of
   kvm_uninit_virtualization(), as it's annoyingly difficult to keep the WARN
   and still allow arch code (i.e. TDX) to enable virtualization during setup.

v3:
 - https://lore.kernel.org/all/20240608000639.3295768-1-seanjc@google.com
 - Collect reviews/acks.
 - Switch to kvm_usage_lock in a dedicated patch, Cc'd for stable@. [Chao]
 - Enable virt at load by default. [Chao]
 - Add comments to document how kvm_arch_{en,dis}able_virtualization() fit
   into the overall flow. [Kai]

v2:
 - https://lore.kernel.org/all/20240522022827.1690416-1-seanjc@google.com
 - Use a dedicated mutex to avoid lock inversion issues between kvm_lock and
   the cpuhp lock.
 - Register emergency disable callbacks on-demand. [Kai]
 - Drop an unintended s/junk/ign rename. [Kai]
 - Decrement kvm_usage_count on failure. [Chao]

v1: https://lore.kernel.org/all/20240425233951.3344485-1-seanjc@google.com

Sean Christopherson (10):
  KVM: Use dedicated mutex to protect kvm_usage_count to avoid deadlock
  KVM: Register cpuhp and syscore callbacks when enabling hardware
  KVM: Rename symbols related to enabling virtualization hardware
  KVM: Rename arch hooks related to per-CPU virtualization enabling
  KVM: MIPS: Rename virtualization {en,dis}abling APIs to match common
    KVM
  KVM: x86: Rename virtualization {en,dis}abling APIs to match common
    KVM
  KVM: Add a module param to allow enabling virtualization when KVM is
    loaded
  KVM: Add arch hooks for enabling/disabling virtualization
  x86/reboot: Unconditionally define cpu_emergency_virt_cb typedef
  KVM: x86: Register "emergency disable" callbacks when virt is enabled

 .../admin-guide/kernel-parameters.txt         |  17 ++
 Documentation/virt/kvm/locking.rst            |  31 +-
 arch/arm64/kvm/arm.c                          |   6 +-
 arch/loongarch/kvm/main.c                     |   4 +-
 arch/mips/include/asm/kvm_host.h              |   4 +-
 arch/mips/kvm/mips.c                          |   8 +-
 arch/mips/kvm/vz.c                            |   8 +-
 arch/riscv/kvm/main.c                         |   4 +-
 arch/x86/include/asm/kvm-x86-ops.h            |   4 +-
 arch/x86/include/asm/kvm_host.h               |   7 +-
 arch/x86/include/asm/reboot.h                 |   2 +-
 arch/x86/kvm/svm/svm.c                        |  19 +-
 arch/x86/kvm/vmx/main.c                       |   6 +-
 arch/x86/kvm/vmx/vmx.c                        |  10 +-
 arch/x86/kvm/vmx/x86_ops.h                    |   5 +-
 arch/x86/kvm/x86.c                            |  26 +-
 include/linux/kvm_host.h                      |  18 +-
 virt/kvm/kvm_main.c                           | 270 +++++++++---------
 18 files changed, 251 insertions(+), 198 deletions(-)


base-commit: 15e1c3d65975524c5c792fcd59f7d89f00402261
-- 
2.46.0.469.g59c65b2a67-goog


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