[RFC v1 18/26] x86/topology: Disable CPU hotplug support for TDX platforms.

[Kuppuswamy Sathyanarayanan]

As per Intel TDX Virtual Firmware Design Guide, sec 4.3.5 and
sec 9.4, all unused CPUs are put in spinning state by
TDVF until OS requests for CPU bring-up via mailbox address passed
by ACPI MADT table. Since by default all unused CPUs are always in
spinning state, there is no point in supporting dynamic CPU
online/offline feature. So current generation of TDVF does not
support CPU hotplug feature. It may be supported in next generation.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/tdx.c      | 14 ++++++++++++++
 arch/x86/kernel/topology.c |  3 ++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index 8d1d7555fb56..a36b6ae14942 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -387,6 +387,17 @@ static int tdx_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	return insn.length;
 }
 
+static int tdx_cpu_offline_prepare(unsigned int cpu)
+{
+	/*
+	 * Per Intel TDX Virtual Firmware Design Guide,
+	 * sec 4.3.5 and sec 9.4, Hotplug is not supported
+	 * in TDX platforms. So don't support CPU
+	 * offline feature once its turned on.
+	 */
+	return -EOPNOTSUPP;
+}
+
 void __init tdx_early_init(void)
 {
 	if (!cpuid_has_tdx_guest())
@@ -399,6 +410,9 @@ void __init tdx_early_init(void)
 	pv_ops.irq.safe_halt = tdx_safe_halt;
 	pv_ops.irq.halt = tdx_halt;
 
+	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "tdx:cpu_hotplug",
+			  NULL, tdx_cpu_offline_prepare);
+
 	pr_info("TDX guest is initialized\n");
 }
 
diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index f5477eab5692..d879ea96d79c 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -34,6 +34,7 @@
 #include <linux/irq.h>
 #include <asm/io_apic.h>
 #include <asm/cpu.h>
+#include <asm/tdx.h>
 
 static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);
 
@@ -130,7 +131,7 @@ int arch_register_cpu(int num)
 			}
 		}
 	}
-	if (num || cpu0_hotpluggable)
+	if ((num || cpu0_hotpluggable) && !is_tdx_guest())
 		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
 
 	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
-- 
2.25.1