To run in the VTL mode, Hyper-V drivers have to know what
VTL the system boots in, and the arm64/hyperv code does not
have the means to compute that.

Refactor the code to hoist the function that detects VTL,
make it arch-neutral to be able to employ it to get the VTL
on arm64. Fix the hypercall output address in `get_vtl(void)`
not to overlap with the hypercall input area to adhere to
the Hyper-V TLFS.

Signed-off-by: Roman Kisel <redacted>
---
 arch/x86/hyperv/hv_init.c          | 34 ---------------------
 arch/x86/include/asm/hyperv-tlfs.h |  7 -----
 drivers/hv/hv_common.c             | 47 ++++++++++++++++++++++++++++--
 include/asm-generic/hyperv-tlfs.h  |  7 +++++
 include/asm-generic/mshyperv.h     |  6 ++++
 5 files changed, 58 insertions(+), 43 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 17a71e92a343..c350fa05ee59 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c

@@ -413,40 +413,6 @@ static void __init hv_get_partition_id(void)
 	local_irq_restore(flags);
 }

-#if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
-static u8 __init get_vtl(void)
-{
-	u64 control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_REGISTERS;
-	struct hv_get_vp_registers_input *input;
-	struct hv_get_vp_registers_output *output;
-	unsigned long flags;
-	u64 ret;
-
-	local_irq_save(flags);
-	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
-	output = (struct hv_get_vp_registers_output *)input;
-
-	memset(input, 0, struct_size(input, element, 1));
-	input->header.partitionid = HV_PARTITION_ID_SELF;
-	input->header.vpindex = HV_VP_INDEX_SELF;
-	input->header.inputvtl = 0;
-	input->element[0].name0 = HV_X64_REGISTER_VSM_VP_STATUS;
-
-	ret = hv_do_hypercall(control, input, output);
-	if (hv_result_success(ret)) {
-		ret = output->as64.low & HV_X64_VTL_MASK;
-	} else {
-		pr_err("Failed to get VTL(error: %lld) exiting...\n", ret);
-		BUG();
-	}
-
-	local_irq_restore(flags);
-	return ret;
-}
-#else
-static inline u8 get_vtl(void) { return 0; }
-#endif
-
 /*
  * This function is to be invoked early in the boot sequence after the
  * hypervisor has been detected.

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 3787d26810c1..9ee68eb8e6ff 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h

@@ -309,13 +309,6 @@ enum hv_isolation_type {
 #define HV_MSR_STIMER0_CONFIG	(HV_X64_MSR_STIMER0_CONFIG)
 #define HV_MSR_STIMER0_COUNT	(HV_X64_MSR_STIMER0_COUNT)

-/*
- * Registers are only accessible via HVCALL_GET_VP_REGISTERS hvcall and
- * there is not associated MSR address.
- */
-#define	HV_X64_REGISTER_VSM_VP_STATUS	0x000D0003
-#define	HV_X64_VTL_MASK			GENMASK(3, 0)
-
 /* Hyper-V memory host visibility */
 enum hv_mem_host_visibility {
 	VMBUS_PAGE_NOT_VISIBLE		= 0,

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index 9c452bfbd571..7d6c1523b0b5 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c

@@ -339,8 +339,8 @@ int __init hv_common_init(void)
 	hyperv_pcpu_input_arg = alloc_percpu(void  *);
 	BUG_ON(!hyperv_pcpu_input_arg);

-	/* Allocate the per-CPU state for output arg for root */
-	if (hv_root_partition) {
+	/* Allocate the per-CPU state for output arg for root or a VTL */
+	if (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) {
 		hyperv_pcpu_output_arg = alloc_percpu(void *);
 		BUG_ON(!hyperv_pcpu_output_arg);
 	}

@@ -656,3 +656,46 @@ u64 __weak hv_tdx_hypercall(u64 control, u64 param1, u64

param2)
 	return HV_STATUS_INVALID_PARAMETER;
 }
 EXPORT_SYMBOL_GPL(hv_tdx_hypercall);
+
+#if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
+u8 __init get_vtl(void)
+{
+	u64 control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_REGISTERS;
+	struct hv_get_vp_registers_input *input;
+	struct hv_get_vp_registers_output *output;
+	unsigned long flags;
+	u64 ret;
+
+	local_irq_save(flags);
+	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
+	output = *this_cpu_ptr(hyperv_pcpu_output_arg);

+
+	memset(input, 0, struct_size(input, element, 1));
+	input->header.partitionid = HV_PARTITION_ID_SELF;
+	input->header.vpindex = HV_VP_INDEX_SELF;
+	input->header.inputvtl = 0;
+	input->element[0].name0 = HV_REGISTER_VSM_VP_STATUS;
+
+	ret = hv_do_hypercall(control, input, output);
+	if (hv_result_success(ret)) {
+		ret = output->as64.low & HV_VTL_MASK;
+	} else {
+		pr_err("Failed to get VTL(error: %lld) exiting...\n", ret);
+
+		/*
+		 * This is a dead end, something fundamental is broken.
+		 *
+		 * There is no sensible way of continuing as the Hyper-V drivers
+		 * transitively depend via the vmbus driver on knowing which VTL
+		 * they run in to establish communication with the host. The kernel
+		 * is going to be worse off if continued booting than a panicked one,
+		 * just hung and stuck, producing second-order failures, with neither
+		 * a way to recover nor to provide expected services.
+		 */
+		BUG();
+	}
+
+	local_irq_restore(flags);
+	return ret;
+}
+#endif

diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
index 814207e7c37f..271c365973d6 100644
--- a/include/asm-generic/hyperv-tlfs.h
+++ b/include/asm-generic/hyperv-tlfs.h

@@ -75,6 +75,13 @@
 /* AccessTscInvariantControls privilege */
 #define HV_ACCESS_TSC_INVARIANT			BIT(15)

+/*
+ * This synthetic register is only accessible via the HVCALL_GET_VP_REGISTERS
+ * hvcall, and there is no an associated MSR on x86.

+
 /*
  * Group B features.
  */

diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index 8fe7aaab2599..85a5b8cb1702 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h

@@ -315,4 +315,10 @@ static inline enum hv_isolation_type

hv_get_isolation_type(void)
 }
 #endif /* CONFIG_HYPERV */

+#if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
+u8 __init get_vtl(void);
+#else
+static inline u8 get_vtl(void) { return 0; }
+#endif
+
 #endif
--
2.34.1