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From:   Jinank Jain <jinankjain@linux.microsoft.com>
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        linux-arch@vger.kernel.org
Subject: [PATCH 1/6] mshv: Add support for detecting nested hypervisor
Date:   Wed,  2 Nov 2022 14:00:12 +0000
Message-Id: <8f7173a7454f0a87d9bc0afccf9857851a804901.1667394408.git.jinankjain@microsoft.com>
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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@linux.microsoft.com>
---
 arch/x86/include/asm/hyperv-tlfs.h | 3 +++
 arch/x86/kernel/cpu/mshyperv.c     | 7 +++++++
 include/asm-generic/mshyperv.h     | 2 ++
 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/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.
 	 */
diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index bfb9eb9d7215..49d2e9274379 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -115,6 +115,8 @@ static inline u64 hv_generate_guest_id(u64 kernel_version)
 	return guest_id;
 }
 
+extern bool hv_nested;
+
 /* Free the message slot and signal end-of-message if required */
 static inline void vmbus_signal_eom(struct hv_message *msg, u32 old_msg_type)
 {
-- 
2.25.1