[PATCH v5 1/5] x86/hyperv: Add support for detecting nested hypervisor

From: Jinank Jain
Date: Thu Nov 24 2022 - 00:53:27 EST


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/arm64/hyperv/mshyperv.c | 6 ++++++
arch/x86/include/asm/hyperv-tlfs.h | 3 +++
arch/x86/kernel/cpu/mshyperv.c | 7 +++++++
drivers/hv/hv_common.c | 7 +++++--
include/asm-generic/mshyperv.h | 1 +
5 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
index a406454578f0..2024b19dc514 100644
--- a/arch/arm64/hyperv/mshyperv.c
+++ b/arch/arm64/hyperv/mshyperv.c
@@ -19,6 +19,9 @@

static bool hyperv_initialized;

+/* Is Linux running on nested Microsoft Hypervisor */
+bool hv_nested;
+
static int __init hyperv_init(void)
{
struct hv_get_vp_registers_output result;
@@ -63,6 +66,9 @@ static int __init hyperv_init(void)
pr_info("Hyper-V: Host Build %d.%d.%d.%d-%d-%d\n",
b >> 16, b & 0xFFFF, a, d & 0xFFFFFF, c, d >> 24);

+ /* ARM64 does not support nested virtualization */
+ hv_nested = false;
+
ret = hv_common_init();
if (ret)
return ret;
diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 6d9368ea3701..58c03d18c235 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..9a4204139490 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: running on a nested hypervisor\n");
+ }
+
/*
* Extract host information.
*/
diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index ae68298c0dca..dcb336ce374f 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -25,8 +25,8 @@
#include <asm/mshyperv.h>

/*
- * hv_root_partition and ms_hyperv are defined here with other Hyper-V
- * specific globals so they are shared across all architectures and are
+ * hv_root_partition, ms_hyperv and hv_nested are defined here with other
+ * Hyper-V specific globals so they are shared across all architectures and are
* built only when CONFIG_HYPERV is defined. But on x86,
* ms_hyperv_init_platform() is built even when CONFIG_HYPERV is not
* defined, and it uses these two variables. So mark them as __weak
@@ -36,6 +36,9 @@
bool __weak hv_root_partition;
EXPORT_SYMBOL_GPL(hv_root_partition);

+bool __weak hv_nested;
+EXPORT_SYMBOL_GPL(hv_nested);
+
struct ms_hyperv_info __weak ms_hyperv;
EXPORT_SYMBOL_GPL(ms_hyperv);

diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index bfb9eb9d7215..5df6e944e6a9 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -164,6 +164,7 @@ extern int vmbus_interrupt;
extern int vmbus_irq;

extern bool hv_root_partition;
+extern bool hv_nested;

#if IS_ENABLED(CONFIG_HYPERV)
/*
--
2.25.1