KVM: x86: Clear hardware HLT state when userspace makes a vCPU not-halted

Force a vCPU out of its hardware-tracked halted state when userspace puts
the vCPU into any state other than HALTED via KVM_SET_MP_STATE, i.e. clear
VMCS.GUEST_ACTIVITY_STATE if it says the vCPU is halted.  Add an optional
kvm_x86_ops hook to do the clearing, as SVM has no equivalent VMCB field.

When HLT-exiting is disabled for a VM (KVM_CAP_X86_DISABLE_EXITS with
KVM_X86_DISABLE_EXITS_HLT, e.g. QEMU's "-overcommit cpu-pm=on"), a guest
HLT halts the physical CPU instead of exiting to KVM, and hardware saves
GUEST_ACTIVITY_STATE=HLT into the VMCS on the next VM-Exit.  That field is
sticky: it survives VM-Exit/VM-Enter and is only cleared by vmx_clear_hlt()
on event injection, or by vmx_vcpu_reset() on INIT / vCPU creation.

Nothing clears it on a userspace-driven state change.  KVM_SET_REGS only
writes the software register cache and KVM_SET_MP_STATE only writes
vcpu->arch.mp_state; kvm_vcpu_running() likewise consults software state
only.  A VMM that emulates a machine reset therefore ends up with a vCPU
that KVM happily VM-Enters while hardware refuses to fetch instructions.

Reproduce with a Linux guest by triggering a panic/kdump on a non-boot
vCPU: nmi_shootdown_cpus() parks the other vCPUs -- including vCPU0 -- in
crash_nmi_callback(), which does local_irq_disable() followed by a bare
HLT.  The capture kernel then resets the machine via port 0xCF9.  QEMU
rewrites RIP to 0xfff0 and sets mp_state to RUNNABLE, but vCPU0's
GUEST_ACTIVITY_STATE is still HLT, so the BSP never executes the reset
vector, never sends SIPIs, and the entire VM hangs at "reboot: machine
restart" forever.  Only destroying and recreating the VM recovers it.

Key off "not HALTED" rather than "is RUNNABLE" as HALTED is the only target
state for which retaining the hardware halted state is meaningful; every
other transition either can't be reached on VMX, e.g. AP_RESET_HOLD, or
goes through vmx_vcpu_reset(), which stuffs GUEST_ACTIVITY_ACTIVE anyways.
This also captures the intent more directly: KVM only ever drops the
hardware halted state, it never installs it.

Hook KVM_SET_MP_STATE rather than the VM-Enter path so that the clearing
is driven by an explicit userspace declaration, and so that no work is
added to vmx_vcpu_run().  Gate the call on kvm_hlt_in_guest() even though
vmx_clear_hlt() checks it as well, as the check is cheap and makes it
obvious in common code when the hardware state can exist at all.

Do nothing for TDX vCPUs as TDX doesn't support disabling HLT-exiting and
KVM cannot access a TD's VMCS, i.e. the VMCS used to run the vCPU can't
in HALTED state, and even if it were, KVM can't change it.

Fixes: caa057a2cad6 ("KVM: X86: Provide a capability to disable HLT intercepts")
Cc: stable@vger.kernel.org
Tested-by: Qinguang Chen <chenqinguang@kuaishou.com>
Signed-off-by: Zhiping Du <duzhiping@kuaishou.com>
Signed-off-by: Keqiang Duan <duankeqiangcym@gmail.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20260820123512.87236-1-duankeqiangcym@gmail.com
[sean: drop claim that spurious wakeups are ok, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions