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Subject[RFC v1 18/26] x86/topology: Disable CPU hotplug support for TDX platforms.
Date
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@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
---
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
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