[RFC v2 24/32] x86/topology: Disable CPU online/offline control for TDX guest

From: Kuppuswamy Sathyanarayanan
Date: Mon Apr 26 2021 - 14:04:07 EST


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>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
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 294dda5bf3f6..ab1efa4d10e9 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -316,6 +316,17 @@ static int tdg_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
return insn.length;
}

+static int tdg_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;
+}
+
unsigned long tdg_get_ve_info(struct ve_info *ve)
{
u64 ret;
@@ -410,5 +421,8 @@ void __init tdx_early_init(void)
pv_ops.irq.safe_halt = tdg_safe_halt;
pv_ops.irq.halt = tdg_halt;

+ cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "tdg:cpu_hotplug",
+ NULL, tdg_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