x86/cpu: Detect FreeBSD Bhyve hypervisor

Detect the Bhyve hypervisor and enable 15-bit MSI support if available.

Detecting Bhyve used to be a purely cosmetic issue of the kernel printing
'Hypervisor detected: Bhyve' at boot time.

But FreeBSD 15.0 will support¹ the 15-bit MSI enlightenment to support
more than 255 vCPUs (http://david.woodhou.se/ExtDestId.pdf) which means
there's now actually some functional reason to do so.

  ¹ https://github.com/freebsd/freebsd-src/commit/313a68ea20b4

  [ bp: Massage, move tail comment ontop. ]

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Ahmed S. Darwish <darwi@linutronix.de>
Link: https://lore.kernel.org/03802f6f7f5b5cf8c5e8adfe123c397ca8e21093.camel@infradead.org

3 files changed, 70 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index 1e26179ff18c..2f8a58ef690e 100644
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -58,6 +58,7 @@ obj-$(CONFIG_X86_SGX)			+= sgx/
 obj-$(CONFIG_X86_LOCAL_APIC)		+= perfctr-watchdog.o
 
 obj-$(CONFIG_HYPERVISOR_GUEST)		+= vmware.o hypervisor.o mshyperv.o
+obj-$(CONFIG_BHYVE_GUEST)		+= bhyve.o
 obj-$(CONFIG_ACRN_GUEST)		+= acrn.o
 
 obj-$(CONFIG_DEBUG_FS)			+= debugfs.o
diff --git a/arch/x86/kernel/cpu/bhyve.c b/arch/x86/kernel/cpu/bhyve.c
new file mode 100644
index 000000000000..f1a8ca3dd1ed
--- /dev/null
+++ b/arch/x86/kernel/cpu/bhyve.c
@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * FreeBSD Bhyve guest enlightenments
+ *
+ * Copyright © 2025 Amazon.com, Inc. or its affiliates.
+ *
+ * Author: David Woodhouse <dwmw2@infradead.org>
+ */
+
+#include <linux/init.h>
+#include <linux/export.h>
+#include <asm/processor.h>
+#include <asm/hypervisor.h>
+
+static uint32_t bhyve_cpuid_base;
+static uint32_t bhyve_cpuid_max;
+
+#define BHYVE_SIGNATURE			"bhyve bhyve "
+
+#define CPUID_BHYVE_FEATURES		0x40000001
+
+/* Features advertised in CPUID_BHYVE_FEATURES %eax */
+
+/* MSI Extended Dest ID */
+#define CPUID_BHYVE_FEAT_EXT_DEST_ID	(1UL << 0)
+
+static uint32_t __init bhyve_detect(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+                return 0;
+
+	bhyve_cpuid_base = cpuid_base_hypervisor(BHYVE_SIGNATURE, 0);
+	if (!bhyve_cpuid_base)
+		return 0;
+
+	bhyve_cpuid_max = cpuid_eax(bhyve_cpuid_base);
+	return bhyve_cpuid_max;
+}
+
+static uint32_t bhyve_features(void)
+{
+	unsigned int cpuid_leaf = bhyve_cpuid_base | CPUID_BHYVE_FEATURES;
+
+	if (bhyve_cpuid_max < cpuid_leaf)
+		return 0;
+
+	return cpuid_eax(cpuid_leaf);
+}
+
+static bool __init bhyve_ext_dest_id(void)
+{
+	return !!(bhyve_features() & CPUID_BHYVE_FEAT_EXT_DEST_ID);
+}
+
+static bool __init bhyve_x2apic_available(void)
+{
+	return true;
+}
+
+const struct hypervisor_x86 x86_hyper_bhyve __refconst = {
+	.name			= "Bhyve",
+	.detect			= bhyve_detect,
+	.init.init_platform	= x86_init_noop,
+	.init.x2apic_available	= bhyve_x2apic_available,
+	.init.msi_ext_dest_id	= bhyve_ext_dest_id,
+};
diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
index 553bfbfc3a1b..f3e9219845e8 100644
--- a/arch/x86/kernel/cpu/hypervisor.c
+++ b/arch/x86/kernel/cpu/hypervisor.c
@@ -45,6 +45,9 @@ static const __initconst struct hypervisor_x86 * const hypervisors[] =
 #ifdef CONFIG_ACRN_GUEST
 	&x86_hyper_acrn,
 #endif
+#ifdef CONFIG_BHYVE_GUEST
+	&x86_hyper_bhyve,
+#endif
 };
 
 enum x86_hypervisor_type x86_hyper_type;