[PATCH v4 02/10] x86/tdx: Add early_is_tdx_guest() interface

[Kuppuswamy Sathyanarayanan]

Add helper function to detect TDX feature support. It will be used
to protect TDX specific code in decompression code (for example to
add TDX specific I/O fixes in decompression code).

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---

Changes since v3:
 * None

Changes since v2:
 * Fixed string order issue in cpuid_count() call.

Changes since v1:
 * Modified cpuid_has_tdx_guest() to use cpuid_count() instead of native_cpuid().
 * Reused cpuid_count() from cpuflags.c.
 * Added a new function cpuid_eax().
 * Renamed native_cpuid_has_tdx_guest() as early_cpuid_has_tdx_guest().

 arch/x86/boot/compressed/Makefile |  1 +
 arch/x86/boot/compressed/tdx.c    | 31 +++++++++++++++++++++++++++++++
 arch/x86/boot/cpuflags.c          | 12 ++++++++++--
 arch/x86/boot/cpuflags.h          |  2 ++
 4 files changed, 44 insertions(+), 2 deletions(-)
 create mode 100644 arch/x86/boot/compressed/tdx.c

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 431bf7f846c3..22a2a6cc2ab4 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -98,6 +98,7 @@ 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..ecb3b42992e0
--- /dev/null
+++ b/arch/x86/boot/compressed/tdx.c
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * tdx.c - Early boot code for TDX
+ */
+
+#include "../cpuflags.h"
+#include "../string.h"
+
+#define TDX_CPUID_LEAF_ID                       0x21
+
+static int tdx_guest = -1;
+
+static inline bool early_cpuid_has_tdx_guest(void)
+{
+	u32 eax = TDX_CPUID_LEAF_ID, sig[3] = {0};
+
+	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID)
+		return false;
+
+	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
+
+	return !memcmp("IntelTDX    ", sig, 12);
+}
+
+bool early_is_tdx_guest(void)
+{
+	if (tdx_guest < 0)
+		tdx_guest = early_cpuid_has_tdx_guest();
+
+	return !!tdx_guest;
+}
diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
index a0b75f73dc63..102613a092aa 100644
--- a/arch/x86/boot/cpuflags.c
+++ b/arch/x86/boot/cpuflags.c
@@ -71,8 +71,7 @@ int has_eflag(unsigned long mask)
 # define EBX_REG "=b"
 #endif
 
-static inline void cpuid_count(u32 id, u32 count,
-		u32 *a, u32 *b, u32 *c, u32 *d)
+void cpuid_count(u32 id, u32 count, u32 *a, u32 *b, u32 *c, u32 *d)
 {
 	asm volatile(".ifnc %%ebx,%3 ; movl  %%ebx,%3 ; .endif	\n\t"
 		     "cpuid					\n\t"
@@ -82,6 +81,15 @@ static inline void cpuid_count(u32 id, u32 count,
 	);
 }
 
+u32 cpuid_eax(u32 id)
+{
+	u32 eax, ebx, ecx, edx;
+
+	cpuid_count(id, 0, &eax, &ebx, &ecx, &edx);
+
+	return eax;
+}
+
 #define cpuid(id, a, b, c, d) cpuid_count(id, 0, a, b, c, d)
 
 void get_cpuflags(void)
diff --git a/arch/x86/boot/cpuflags.h b/arch/x86/boot/cpuflags.h
index 2e20814d3ce3..5a72233eb8fd 100644
--- a/arch/x86/boot/cpuflags.h
+++ b/arch/x86/boot/cpuflags.h
@@ -17,5 +17,7 @@ extern u32 cpu_vendor[3];
 
 int has_eflag(unsigned long mask);
 void get_cpuflags(void);
+void cpuid_count(u32 id, u32 count, u32 *a, u32 *b, u32 *c, u32 *d);
+u32 cpuid_eax(u32 id);
 
 #endif
-- 
2.25.1