Re: [PATCH] KVM: VMX: Clear GUEST_ACTIVITY_STATE when userspace makes a vCPU RUNNABLE

[Chao Gao]

On Wed, Aug 19, 2026 at 11:46:52AM +0800, Keqiang Duan wrote:
>Force a vCPU out of its hardware-tracked halted state when userspace
>explicitly declares the vCPU RUNNABLE 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.
>
>Clearing the state is always safe: waking from HLT is architecturally
>permitted to be spurious, and every HLT in the kernel is inside a loop.
>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().
>
>Note, vmx_clear_hlt() loses its "static" qualifier as the kvm_x86_ops table
>now lives in vmx/main.c.  TDX cannot disable HLT-exiting and KVM cannot
>access a TD's VMCS, so vt_clear_hlt() short-circuits for TD vCPUs,
>following the existing vt_*() wrapper pattern.
>
>Fixes: caa057a2cad6 ("KVM: X86: Provide a capability to disable HLT intercepts")
>Cc: stable@xxxxxxxxxxxxxxx
>Signed-off-by: Keqiang Duan <duankeqiangcym@xxxxxxxxx>

There was an earlier attempt to fix this issue:

https://lore.kernel.org/kvm/20230630072612.1106705-1-aiqi.i7@xxxxxxxxxxxxx

Sean suggested doing exactly this there, i.e. clearing the activity state in
kvm_arch_vcpu_ioctl_set_mpstate() via a new kvm_x86_ops hook:

https://lore.kernel.org/kvm/ZMgIQ5m1jMSAogT4@xxxxxxxxxx/