[PATCH v7 3/6] x86/topology: Disable CPU online/offline control for TDX guest

[Kuppuswamy Sathyanarayanan]

As per Intel TDX Virtual Firmware Design Guide, sec titled "AP
initialization in OS" (index 4.3.5) and sec titled "Hotplug Device"
(index 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>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---

Changes since v6:
 * None

 arch/x86/kernel/tdx.c      | 16 ++++++++++++++++
 arch/x86/kernel/topology.c |  3 ++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index a66520405109..203deb57c4c9 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -4,6 +4,8 @@
 #undef pr_fmt
 #define pr_fmt(fmt)     "tdx: " fmt
 
+#include <linux/cpuhotplug.h>
+
 #include <asm/tdx.h>
 #include <asm/vmx.h>
 #include <asm/insn.h>
@@ -307,6 +309,17 @@ 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 it is turned on.
+	 */
+	return -EOPNOTSUPP;
+}
+
 unsigned long tdx_get_ve_info(struct ve_info *ve)
 {
 	struct tdx_module_output out = {0};
@@ -451,5 +464,8 @@ 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("Guest initialized\n");
 }
diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index bd83748e2bde..ded34eda5bac 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -32,6 +32,7 @@
 #include <linux/init.h>
 #include <linux/smp.h>
 #include <linux/irq.h>
+#include <linux/cc_platform.h>
 #include <asm/io_apic.h>
 #include <asm/cpu.h>
 
@@ -130,7 +131,7 @@ int arch_register_cpu(int num)
 			}
 		}
 	}
-	if (num || cpu0_hotpluggable)
+	if ((num || cpu0_hotpluggable) && !cc_platform_has(CC_ATTR_GUEST_TDX))
 		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
 
 	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
-- 
2.25.1