[PATCH v3 0/8] KVM: Register cpuhp/syscore callbacks when enabling virt

[Sean Christopherson]

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.  TDX
doesn't _need_ to keep virtualization enabled, but doing so is much simpler
for KVM (see patch 3).

That said, this is a nice cleanup on its own, assuming I haven't broken
something.  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.

The meat is in patch 1.  Patches 2 renames the helpers so that patch 3 is
less awkward.  Patch 3 adds a module param to enable virtualization when KVM
is loaded.  Patches 4-6 are tangentially related x86 cleanups to registers
KVM's "emergency disable" callback on-demand, same as the syscore callbacks.

The suspend/resume and cphup paths still need to be fully tested, as do
non-x86 architectures.

v3:
 - 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@xxxxxxxxxx
 - 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@xxxxxxxxxx

Sean Christopherson (8):
  KVM: Use dedicated mutex to protect kvm_usage_count to avoid deadlock
  KVM: Register cpuhp and syscore callbacks when enabling hardware
  KVM: Rename functions related to enabling virtualization hardware
  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
  KVM: Enable virtualization at load/initialization by default

 Documentation/virt/kvm/locking.rst |  19 ++-
 arch/x86/include/asm/kvm_host.h    |   3 +
 arch/x86/include/asm/reboot.h      |   2 +-
 arch/x86/kvm/svm/svm.c             |   5 +-
 arch/x86/kvm/vmx/main.c            |   2 +
 arch/x86/kvm/vmx/vmx.c             |   6 +-
 arch/x86/kvm/vmx/x86_ops.h         |   1 +
 arch/x86/kvm/x86.c                 |  10 ++
 include/linux/kvm_host.h           |  14 ++
 virt/kvm/kvm_main.c                | 258 ++++++++++++++---------------
 10 files changed, 175 insertions(+), 145 deletions(-)


base-commit: af0903ab52ee6d6f0f63af67fa73d5eb00f79b9a
-- 
2.45.2.505.gda0bf45e8d-goog