[RFC v2 04/32] x86/x86: Add is_tdx_guest() interface

[Kuppuswamy Sathyanarayanan]

Add helper function to detect TDX feature support. It will be used
to protect TDX specific code.

Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/Makefile |  1 +
 arch/x86/boot/compressed/tdx.c    | 32 +++++++++++++++++++++++++++++++
 arch/x86/include/asm/tdx.h        |  8 ++++++++
 arch/x86/kernel/tdx.c             |  6 ++++++
 4 files changed, 47 insertions(+)
 create mode 100644 arch/x86/boot/compressed/tdx.c

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index e0bc3988c3fa..a2554621cefe 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -96,6 +96,7 @@ ifdef CONFIG_X86_64
 endif
 
 vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
+vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdx.o
 
 vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
 efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
diff --git a/arch/x86/boot/compressed/tdx.c b/arch/x86/boot/compressed/tdx.c
new file mode 100644
index 000000000000..0a87c1775b67
--- /dev/null
+++ b/arch/x86/boot/compressed/tdx.c
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * tdx.c - Early boot code for TDX
+ */
+
+#include <asm/tdx.h>
+
+static int __ro_after_init tdx_guest = -1;
+
+static inline bool native_cpuid_has_tdx_guest(void)
+{
+	u32 eax = TDX_CPUID_LEAF_ID, signature[3] = {0};
+
+	if (native_cpuid_eax(0) < TDX_CPUID_LEAF_ID)
+		return false;
+
+	native_cpuid(&eax, &signature[0], &signature[1], &signature[2]);
+
+	if (memcmp("IntelTDX    ", signature, 12))
+		return false;
+
+	return true;
+}
+
+bool is_tdx_guest(void)
+{
+	if (tdx_guest < 0)
+		tdx_guest = native_cpuid_has_tdx_guest();
+
+	return !!tdx_guest;
+}
+
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 679500e807f3..69af72d08d3d 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -9,10 +9,18 @@
 
 #include <asm/cpufeature.h>
 
+/* Common API to check TDX support in decompression and common kernel code. */
+bool is_tdx_guest(void);
+
 void __init tdx_early_init(void);
 
 #else // !CONFIG_INTEL_TDX_GUEST
 
+static inline bool is_tdx_guest(void)
+{
+	return false;
+}
+
 static inline void tdx_early_init(void) { };
 
 #endif /* CONFIG_INTEL_TDX_GUEST */
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index f927e36769d5..6a7193fead08 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -19,6 +19,12 @@ static inline bool cpuid_has_tdx_guest(void)
 	return true;
 }
 
+bool is_tdx_guest(void)
+{
+	return static_cpu_has(X86_FEATURE_TDX_GUEST);
+}
+EXPORT_SYMBOL_GPL(is_tdx_guest);
+
 void __init tdx_early_init(void)
 {
 	if (!cpuid_has_tdx_guest())
-- 
2.25.1