[PATCH v3] x86/bhyve: Detect FreeBSD Bhyve hypervisor

[PATCH v3] x86/bhyve: Detect FreeBSD Bhyve hypervisor

[David Woodhouse]

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From: David Woodhouse <dwmw@amazon.co.uk>

This detects the Bhyve hypervisor and enables 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

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
I still haven't been able to test this properly as I cannot persuade
Bhyve to boot any guest at all.

v3:
 • Drop the cpuid_level check
 • Define and use BHYVE_SIGNATURE
 • Use the actual definition of CPUID_BHYVE_FEATURES and logical OR
   instead of adding, as we do for other hypervisor leaves elsewhere
 • Fix bhyve_cpuid_max miscalculation mistake

v2:
 • Make it optional with CONFIG_BHYVE_GUEST
 • Use cpu_feature_enabled()

 arch/x86/Kconfig                  |  9 +++++
 arch/x86/include/asm/hypervisor.h |  2 +
 arch/x86/kernel/cpu/Makefile      |  1 +
 arch/x86/kernel/cpu/bhyve.c       | 63 +++++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/hypervisor.c  |  3 ++
 5 files changed, 78 insertions(+)
 create mode 100644 arch/x86/kernel/cpu/bhyve.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 58d890fe2100..ac1c6df44212 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -879,6 +879,15 @@ config ACRN_GUEST
 	  IOT with small footprint and real-time features. More details can be
 	  found in https://projectacrn.org/.
 
+config BHYVE_GUEST
+	bool "Bhyve (BSD Hypervisor) Guest support"
+	depends on X86_64
+	help
+	  This option allows to run Linux to recognise when it is running as a
+	  guest in the Bhyve hypervisor, and to support more than 255 vCPUs when
+	  when doing so. More details about Bhyve can be found at https://bhyve.org
+	  and https://wiki.freebsd.org/bhyve/.
+
 config INTEL_TDX_GUEST
 	bool "Intel TDX (Trust Domain Extensions) - Guest Support"
 	depends on X86_64 && CPU_SUP_INTEL
diff --git a/arch/x86/include/asm/hypervisor.h b/arch/x86/include/asm/hypervisor.h
index e41cbf2ec41d..9ad86a7d13f6 100644
--- a/arch/x86/include/asm/hypervisor.h
+++ b/arch/x86/include/asm/hypervisor.h
@@ -30,6 +30,7 @@ enum x86_hypervisor_type {
 	X86_HYPER_KVM,
 	X86_HYPER_JAILHOUSE,
 	X86_HYPER_ACRN,
+	X86_HYPER_BHYVE,
 };
 
 #ifdef CONFIG_HYPERVISOR_GUEST
@@ -64,6 +65,7 @@ extern const struct hypervisor_x86 x86_hyper_xen_pv;
 extern const struct hypervisor_x86 x86_hyper_kvm;
 extern const struct hypervisor_x86 x86_hyper_jailhouse;
 extern const struct hypervisor_x86 x86_hyper_acrn;
+extern const struct hypervisor_x86 x86_hyper_bhyve;
 extern struct hypervisor_x86 x86_hyper_xen_hvm;
 
 extern bool nopv;
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..a628482dd441
--- /dev/null
+++ b/arch/x86/kernel/cpu/bhyve.c
@@ -0,0 +1,63 @@
+// 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 */
+#define CPUID_BHYVE_FEAT_EXT_DEST_ID	(1UL << 0) /* MSI Extended Dest ID */
+
+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)
+{
+	if (bhyve_cpuid_max < bhyve_cpuid_base | CPUID_BHYVE_FEATURES)
+		return 0;
+
+	return cpuid_eax(bhyve_cpuid_base | CPUID_BHYVE_FEATURES);
+}
+
+static bool __init bhyve_ext_dest_id(void)
+{
+	return !!(bhyve_features() & CPUID_BHYVE_FEAT_EXT_DEST_ID);
+}
+
+static bool __init bhyve_x2apic_available(void)
+{
+	/* Bhyve has always supported x2apic */
+	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;
-- 
2.49.0



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Re: [PATCH v3] x86/bhyve: Detect FreeBSD Bhyve hypervisor

[David Woodhouse]

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On Fri, 2025-08-15 at 14:03 +0100, David Woodhouse wrote:
> 
> +static uint32_t bhyve_features(void)
> +{
> +	if (bhyve_cpuid_max < bhyve_cpuid_base |
> CPUID_BHYVE_FEATURES)

Meh, needs parens around the 'bhyve_cpuid_base | CPUID_BHYVE_FEATURES',
which somehow I didn't see in the first build.

I shall persist in *trying* to get a Bhyve setup which actually manages
to boot a guest (or failing that, getting someone to boot it for me
since Bhyve clearly hates me) before sending v4 with that fixed.


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