KVM: TDX: Use kvm_arch_vcpu.host_debugctl to restore the host's DEBUGCTL

Use the kvm_arch_vcpu.host_debugctl snapshot to restore DEBUGCTL after
running a TD vCPU.  The final TDX series rebase was mishandled, likely due
to commit fb71c79 ("KVM: x86: Snapshot the host's DEBUGCTL in common
x86") deleting the same line of code from vmx.h, i.e. creating a semantic
conflict of sorts, but no syntactic conflict.

Using the version in kvm_vcpu_arch picks up the ulong => u64 fix (which
isn't relevant to TDX) as well as the IRQ fix from commit 189ecdb
("KVM: x86: Snapshot the host's DEBUGCTL after disabling IRQs").

Link: https://lore.kernel.org/all/20250307212053.2948340-10-pbonzini@redhat.com
Cc: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 8af0990 ("KVM: TDX: Save and restore IA32_DEBUGCTL")
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20250610232010.162191-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/common.h

@@ -53,8 +53,6 @@ struct vcpu_vt {
 #ifdef CONFIG_X86_64
 	u64		msr_host_kernel_gs_base;
 #endif
-
-	unsigned long	host_debugctlmsr;
 };
 
 #ifdef CONFIG_KVM_INTEL_TDX

arch/x86/kvm/vmx/tdx.c

@@ -785,8 +785,6 @@ void tdx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	else
 		vt->msr_host_kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE);
 
-	vt->host_debugctlmsr = get_debugctlmsr();
-
 	vt->guest_state_loaded = true;
 }
 
@@ -1062,8 +1060,8 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
 
 	tdx_vcpu_enter_exit(vcpu);
 
-	if (vt->host_debugctlmsr & ~TDX_DEBUGCTL_PRESERVED)
-		update_debugctlmsr(vt->host_debugctlmsr);
+	if (vcpu->arch.host_debugctl & ~TDX_DEBUGCTL_PRESERVED)
+		update_debugctlmsr(vcpu->arch.host_debugctl);
 
 	tdx_load_host_xsave_state(vcpu);
 	tdx->guest_entered = true;