x86/compressed/acpi: Move EFI detection to helper

Future patches for SEV-SNP-validated CPUID will also require early
parsing of the EFI configuration. Incrementally move the related code
into a set of helpers that can be re-used for that purpose.

First, carve out the functionality which determines the EFI environment
type the machine is booting on.

  [ bp: Massage commit message. ]

Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-25-brijesh.singh@amd.com

arch/x86/boot/compressed/Makefile

@@ -103,6 +103,7 @@ endif
 vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
 
 vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
+vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
 efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
 
 $(obj)/vmlinux: $(vmlinux-objs-y) $(efi-obj-y) FORCE

arch/x86/boot/compressed/acpi.c

@@ -87,7 +87,7 @@ static acpi_physical_address kexec_get_rsdp_addr(void)
 	efi_system_table_64_t *systab;
 	struct efi_setup_data *esd;
 	struct efi_info *ei;
-	char *sig;
+	enum efi_type et;
 
 	esd = (struct efi_setup_data *)get_kexec_setup_data_addr();
 	if (!esd)
@@ -98,10 +98,9 @@ static acpi_physical_address kexec_get_rsdp_addr(void)
 		return 0;
 	}
 
-	ei = &boot_params->efi_info;
-	sig = (char *)&ei->efi_loader_signature;
-	if (strncmp(sig, EFI64_LOADER_SIGNATURE, 4)) {
-		debug_putstr("Wrong kexec EFI loader signature.\n");
+	et = efi_get_type(boot_params);
+	if (et != EFI_TYPE_64) {
+		debug_putstr("Unexpected kexec EFI environment (expected 64-bit EFI).\n");
 		return 0;
 	}
 
@@ -122,29 +121,22 @@ static acpi_physical_address efi_get_rsdp_addr(void)
 	unsigned long systab, config_tables;
 	unsigned int nr_tables;
 	struct efi_info *ei;
+	enum efi_type et;
 	bool efi_64;
-	char *sig;
 
-	ei = &boot_params->efi_info;
-	sig = (char *)&ei->efi_loader_signature;
-
-	if (!strncmp(sig, EFI64_LOADER_SIGNATURE, 4)) {
+	et = efi_get_type(boot_params);
+	if (et == EFI_TYPE_64)
 		efi_64 = true;
-	} else if (!strncmp(sig, EFI32_LOADER_SIGNATURE, 4)) {
+	else if (et == EFI_TYPE_32)
 		efi_64 = false;
-	} else {
-		debug_putstr("Wrong EFI loader signature.\n");
+	else
 		return 0;
-	}
 
 	/* Get systab from boot params. */
+	ei = &boot_params->efi_info;
 #ifdef CONFIG_X86_64
 	systab = ei->efi_systab | ((__u64)ei->efi_systab_hi << 32);
 #else
-	if (ei->efi_systab_hi || ei->efi_memmap_hi) {
-		debug_putstr("Error getting RSDP address: EFI system table located above 4GB.\n");
-		return 0;
-	}
 	systab = ei->efi_systab;
 #endif
 	if (!systab)

arch/x86/boot/compressed/efi.c

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Helpers for early access to EFI configuration table.
+ *
+ * Originally derived from arch/x86/boot/compressed/acpi.c
+ */
+
+#include "misc.h"
+#include <linux/efi.h>
+#include <asm/efi.h>
+
+/**
+ * efi_get_type - Given a pointer to boot_params, determine the type of EFI environment.
+ *
+ * @bp:         pointer to boot_params
+ *
+ * Return: EFI_TYPE_{32,64} for valid EFI environments, EFI_TYPE_NONE otherwise.
+ */
+enum efi_type efi_get_type(struct boot_params *bp)
+{
+	struct efi_info *ei;
+	enum efi_type et;
+	const char *sig;
+
+	ei = &bp->efi_info;
+	sig = (char *)&ei->efi_loader_signature;
+
+	if (!strncmp(sig, EFI64_LOADER_SIGNATURE, 4)) {
+		et = EFI_TYPE_64;
+	} else if (!strncmp(sig, EFI32_LOADER_SIGNATURE, 4)) {
+		et = EFI_TYPE_32;
+	} else {
+		debug_putstr("No EFI environment detected.\n");
+		et = EFI_TYPE_NONE;
+	}
+
+#ifndef CONFIG_X86_64
+	/*
+	 * Existing callers like acpi.c treat this case as an indicator to
+	 * fall-through to non-EFI, rather than an error, so maintain that
+	 * functionality here as well.
+	 */
+	if (ei->efi_systab_hi || ei->efi_memmap_hi) {
+		debug_putstr("EFI system table is located above 4GB and cannot be accessed.\n");
+		et = EFI_TYPE_NONE;
+	}
+#endif
+
+	return et;
+}

arch/x86/boot/compressed/misc.h

@@ -176,4 +176,20 @@ void boot_stage2_vc(void);
 
 unsigned long sev_verify_cbit(unsigned long cr3);
 
+enum efi_type {
+	EFI_TYPE_64,
+	EFI_TYPE_32,
+	EFI_TYPE_NONE,
+};
+
+#ifdef CONFIG_EFI
+/* helpers for early EFI config table access */
+enum efi_type efi_get_type(struct boot_params *bp);
+#else
+static inline enum efi_type efi_get_type(struct boot_params *bp)
+{
+	return EFI_TYPE_NONE;
+}
+#endif /* CONFIG_EFI */
+
 #endif /* BOOT_COMPRESSED_MISC_H */