Re: [PATCH v5 02/16] x86/hyperv: detect if Linux is the root partition

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@kernel.org>
---
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 6bf42aed387e..204010350604 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_CPUID_VIRT_STACK_INTERFACE      0x40000081

@@ -110,6 +111,15 @@
 /* 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 f628e3dc150f..c376d191a260 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).