[PATCH v2 1/5] mshv: Add support for detecting nested hypervisor

[Jinank Jain]

When Linux runs as a root partition for Microsoft Hypervisor. It is
possible to detect if it is running as nested hypervisor using
hints exposed by mshv. While at it expose a new variable called
hv_nested which can be used later for making decisions specific to
nested use case.

Signed-off-by: Jinank Jain <jinankjain@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/hyperv-tlfs.h | 3 +++
 arch/x86/include/asm/mshyperv.h    | 2 ++
 arch/x86/kernel/cpu/mshyperv.c     | 7 +++++++
 3 files changed, 12 insertions(+)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 3089ec352743..d9a611565859 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h
@@ -114,6 +114,9 @@
 /* Recommend using the newer ExProcessorMasks interface */
 #define HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED		BIT(11)
 
+/* Indicates that the hypervisor is nested within a Hyper-V partition. */
+#define HV_X64_HYPERV_NESTED				BIT(12)
+
 /* Recommend using enlightened VMCS */
 #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED		BIT(14)
 
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 61f0c206bff0..29388567eafd 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -190,6 +190,8 @@ static inline void hv_ghcb_terminate(unsigned int set, unsigned int reason) {}
 
 extern bool hv_isolation_type_snp(void);
 
+extern bool hv_nested;
+
 static inline bool hv_is_synic_reg(unsigned int reg)
 {
 	if ((reg >= HV_REGISTER_SCONTROL) &&
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 831613959a92..2555535f5237 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -37,6 +37,8 @@
 
 /* Is Linux running as the root partition? */
 bool hv_root_partition;
+/* Is Linux running on nested Microsoft Hypervisor */
+bool hv_nested;
 struct ms_hyperv_info ms_hyperv;
 
 #if IS_ENABLED(CONFIG_HYPERV)
@@ -301,6 +303,11 @@ static void __init ms_hyperv_init_platform(void)
 		pr_info("Hyper-V: running as root partition\n");
 	}
 
+	if (ms_hyperv.hints & HV_X64_HYPERV_NESTED) {
+		hv_nested = true;
+		pr_info("Hyper-V: Linux running on a nested hypervisor\n");
+	}
+
 	/*
 	 * Extract host information.
 	 */
-- 
2.25.1