From: Mark Rutland <mark.rutland@arm.com> Sent: Monday, August 2, 2021 9:26 AM

On Tue, Jul 20, 2021 at 07:57:01AM -0700, Michael Kelley wrote:

quoted

Add ARM64-specific code to initialize the Hyper-V
hypervisor when booting as a guest VM.

This code is built only when CONFIG_HYPERV is enabled.

Signed-off-by: Michael Kelley <redacted>
---
 arch/arm64/hyperv/Makefile   |  2 +-
 arch/arm64/hyperv/mshyperv.c | 83 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm64/hyperv/mshyperv.c

diff --git a/arch/arm64/hyperv/Makefile b/arch/arm64/hyperv/Makefile
index 1697d30..87c31c0 100644
--- a/arch/arm64/hyperv/Makefile
+++ b/arch/arm64/hyperv/Makefile

@@ -1,2 +1,2 @@
 # SPDX-License-Identifier: GPL-2.0
-obj-y		:= hv_core.o
+obj-y		:= hv_core.o mshyperv.o

diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
new file mode 100644
index 0000000..2811fd0
--- /dev/null
+++ b/arch/arm64/hyperv/mshyperv.c

@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Core routines for interacting with Microsoft's Hyper-V hypervisor,
+ * including hypervisor initialization.
+ *
+ * Copyright (C) 2021, Microsoft, Inc.
+ *
+ * Author : Michael Kelley <mikelley@microsoft.com>
+ */
+
+#include <linux/types.h>
+#include <linux/acpi.h>
+#include <linux/export.h>
+#include <linux/errno.h>
+#include <linux/version.h>
+#include <linux/cpuhotplug.h>
+#include <asm/mshyperv.h>
+
+static bool hyperv_initialized;
+
+static int __init hyperv_init(void)
+{
+	struct hv_get_vp_registers_output	result;
+	u32	a, b, c, d;
+	u64	guest_id;
+	int	ret;

As Marc suggests, before looking at the FADT, you need something like:

	/*
	 * Hyper-V VMs always have ACPI.
	 */
	if (acpi_disabled)
		return 0;

... where `acpi_disabled` is defined in <linux/acpi.h> (or via its
includes), so you don't need to include any additional headers.

quoted

+
+	/*
+	 * If we're in a VM on Hyper-V, the ACPI hypervisor_id field will
+	 * have the string "MsHyperV".
+	 */
+	if (strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8))
+		return -EINVAL;

As Marc suggests, it's no an error for a platform to not have Hyper-V,
so returning 0 in tihs case would be preferable.

Otherwise this looks fine to me.

Thanks Marc and Mark.  Good point that the code should cleanly
handle the case where a kernel is built with CONFIG_HYPERV but
running somewhere other than as a Hyper-V guest.

Michael