[PATCH v3] x86/hyperv: Use per cpu initial stack for vtl context

[Saurabh Sengar <ssengar@linux.microsoft.com>]

Currently, the secondary CPUs in Hyper-V VTL context lack support for
parallel startup. Therefore, relying on the single initial_stack fetched
from the current task structure suffices for all vCPUs.

However, common initial_stack risks stack corruption when parallel startup
is enabled. In order to facilitate parallel startup, use the initial_stack
from the per CPU idle thread instead of the current task.

Fixes: 18415f33e2ac ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE")
Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
---
[V3]
 - Added the VTL code dependency on SMP to fix kernel build error
   when SMP is disabled.

 arch/x86/hyperv/hv_vtl.c | 19 +++++++++++++++----
 drivers/hv/Kconfig       |  1 +
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 804b629ea49d..b4e233954d0f 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -12,6 +12,7 @@
 #include <asm/i8259.h>
 #include <asm/mshyperv.h>
 #include <asm/realmode.h>
+#include <../kernel/smpboot.h>
 
 extern struct boot_params boot_params;
 static struct real_mode_header hv_vtl_real_mode_header;
@@ -58,7 +59,7 @@ static void hv_vtl_ap_entry(void)
 	((secondary_startup_64_fn)secondary_startup_64)(&boot_params, &boot_params);
 }
 
-static int hv_vtl_bringup_vcpu(u32 target_vp_index, u64 eip_ignored)
+static int hv_vtl_bringup_vcpu(u32 target_vp_index, int cpu, u64 eip_ignored)
 {
 	u64 status;
 	int ret = 0;
@@ -72,7 +73,9 @@ static int hv_vtl_bringup_vcpu(u32 target_vp_index, u64 eip_ignored)
 	struct ldttss_desc *ldt;
 	struct desc_struct *gdt;
 
-	u64 rsp = current->thread.sp;
+	struct task_struct *idle = idle_thread_get(cpu);
+	u64 rsp = (unsigned long)idle->thread.sp;
+
 	u64 rip = (u64)&hv_vtl_ap_entry;
 
 	native_store_gdt(&gdt_ptr);
@@ -199,7 +202,15 @@ static int hv_vtl_apicid_to_vp_id(u32 apic_id)
 
 static int hv_vtl_wakeup_secondary_cpu(u32 apicid, unsigned long start_eip)
 {
-	int vp_id;
+	int vp_id, cpu;
+
+	/* Find the logical CPU for the APIC ID */
+	for_each_present_cpu(cpu) {
+		if (arch_match_cpu_phys_id(cpu, apicid))
+			break;
+	}
+	if (cpu >= nr_cpu_ids)
+		return -EINVAL;
 
 	pr_debug("Bringing up CPU with APIC ID %d in VTL2...\n", apicid);
 	vp_id = hv_vtl_apicid_to_vp_id(apicid);
@@ -213,7 +224,7 @@ static int hv_vtl_wakeup_secondary_cpu(u32 apicid, unsigned long start_eip)
 		return -EINVAL;
 	}
 
-	return hv_vtl_bringup_vcpu(vp_id, start_eip);
+	return hv_vtl_bringup_vcpu(vp_id, cpu, start_eip);
 }
 
 int __init hv_vtl_early_init(void)
diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
index 00242107d62e..862c47b191af 100644
--- a/drivers/hv/Kconfig
+++ b/drivers/hv/Kconfig
@@ -16,6 +16,7 @@ config HYPERV
 config HYPERV_VTL_MODE
 	bool "Enable Linux to boot in VTL context"
 	depends on X86_64 && HYPERV
+	depends on SMP
 	default n
 	help
 	  Virtual Secure Mode (VSM) is a set of hypervisor capabilities and
-- 
2.34.1