[PATCH v3 02/17] x86/hyperv: detect if Linux is the root partition

[Wei Liu <wei.liu@xxxxxxxxxx>]

For now we can use the privilege flag to check. Stash the value to be
used later.

Put in a bunch of defines for future use when we want to have more
fine-grained detection.

Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
---
v3: move hv_root_partition to mshyperv.c
---
 arch/x86/include/asm/hyperv-tlfs.h | 10 ++++++++++
 arch/x86/include/asm/mshyperv.h    |  2 ++
 arch/x86/kernel/cpu/mshyperv.c     | 20 ++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 0ed20e8bba9e..41b628b9fb15 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h
@@ -21,6 +21,7 @@
 #define HYPERV_CPUID_FEATURES			0x40000003
 #define HYPERV_CPUID_ENLIGHTMENT_INFO		0x40000004
 #define HYPERV_CPUID_IMPLEMENT_LIMITS		0x40000005
+#define HYPERV_CPUID_CPU_MANAGEMENT_FEATURES	0x40000007
 #define HYPERV_CPUID_NESTED_FEATURES		0x4000000A
 
 #define HYPERV_HYPERVISOR_PRESENT_BIT		0x80000000
@@ -103,6 +104,15 @@
 /* Recommend using enlightened VMCS */
 #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED		BIT(14)
 
+/*
+ * CPU management features identification.
+ * These are HYPERV_CPUID_CPU_MANAGEMENT_FEATURES.EAX bits.
+ */
+#define HV_X64_START_LOGICAL_PROCESSOR			BIT(0)
+#define HV_X64_CREATE_ROOT_VIRTUAL_PROCESSOR		BIT(1)
+#define HV_X64_PERFORMANCE_COUNTER_SYNC			BIT(2)
+#define HV_X64_RESERVED_IDENTITY_BIT			BIT(31)
+
 /*
  * Virtual processor will never share a physical core with another virtual
  * processor, except for virtual processors that are reported as sibling SMT
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index ffc289992d1b..ac2b0d110f03 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -237,6 +237,8 @@ int hyperv_fill_flush_guest_mapping_list(
 		struct hv_guest_mapping_flush_list *flush,
 		u64 start_gfn, u64 end_gfn);
 
+extern bool hv_root_partition;
+
 #ifdef CONFIG_X86_64
 void hv_apic_init(void);
 void __init hv_init_spinlocks(void);
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 05ef1f4550cb..f0b8c702c858 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -32,6 +32,10 @@
 #include <asm/nmi.h>
 #include <clocksource/hyperv_timer.h>
 
+/* Is Linux running as the root partition? */
+bool hv_root_partition;
+EXPORT_SYMBOL_GPL(hv_root_partition);
+
 struct ms_hyperv_info ms_hyperv;
 EXPORT_SYMBOL_GPL(ms_hyperv);
 
@@ -237,6 +241,22 @@ static void __init ms_hyperv_init_platform(void)
 	pr_debug("Hyper-V: max %u virtual processors, %u logical processors\n",
 		 ms_hyperv.max_vp_index, ms_hyperv.max_lp_index);
 
+	/*
+	 * Check CPU management privilege.
+	 *
+	 * To mirror what Windows does we should extract CPU management
+	 * features and use the ReservedIdentityBit to detect if Linux is the
+	 * root partition. But that requires negotiating CPU management
+	 * interface (a process to be finalized).
+	 *
+	 * For now, use the privilege flag as the indicator for running as
+	 * root.
+	 */
+	if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_CPU_MANAGEMENT) {
+		hv_root_partition = true;
+		pr_info("Hyper-V: running as root partition\n");
+	}
+
 	/*
 	 * Extract host information.
 	 */
-- 
2.20.1