[PATCH v15 4/6] x86/boot: Introduce bios_get_rsdp_addr() to search RSDP in memory

[Chao Fan]

Memory information in SRAT table is necessary to fix the conflict
between KASLR and memory-hotremove. So RSDP and SRAT should be parsed.

When booting form KEXEC/EFI/BIOS, the methods to compute RSDP
are different. When booting from BIOS, there is no variable who can
point to RSDP directly, so scan memory for the RSDP and verify RSDP
by signature and checksum.

Signed-off-by: Chao Fan <fanc.fnst@xxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/acpi.c | 86 +++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)

diff --git a/arch/x86/boot/compressed/acpi.c b/arch/x86/boot/compressed/acpi.c
index f74c5d033d79..e9dd84f459ed 100644
--- a/arch/x86/boot/compressed/acpi.c
+++ b/arch/x86/boot/compressed/acpi.c
@@ -111,3 +111,89 @@ static acpi_physical_address efi_get_rsdp_addr(void)
 #endif
 	return rsdp_addr;
 }
+
+static u8 compute_checksum(u8 *buffer, u32 length)
+{
+	u8 *end = buffer + length;
+	u8 sum = 0;
+
+	while (buffer < end)
+		sum += *(buffer++);
+
+	return sum;
+}
+
+/* Search a block of memory for the RSDP signature. */
+static u8 *scan_mem_for_rsdp(u8 *start, u32 length)
+{
+	struct acpi_table_rsdp *rsdp;
+	u8 *address;
+	u8 *end;
+
+	end = start + length;
+
+	/* Search from given start address for the requested length */
+	for (address = start; address < end; address += ACPI_RSDP_SCAN_STEP) {
+		/*
+		 * Both RSDP signature and checksum must be correct.
+		 * Note: Sometimes there exists more than one RSDP in memory;
+		 * the valid RSDP has a valid checksum, all others have an
+		 * invalid checksum.
+		 */
+		rsdp = (struct acpi_table_rsdp *)address;
+
+		/* BAD Signature */
+		if (!ACPI_VALIDATE_RSDP_SIG(rsdp->signature))
+			continue;
+
+		/* Check the standard checksum */
+		if (compute_checksum((u8 *)rsdp, ACPI_RSDP_CHECKSUM_LENGTH))
+			continue;
+
+		/* Check extended checksum if table version >= 2 */
+		if ((rsdp->revision >= 2) &&
+		    (compute_checksum((u8 *)rsdp, ACPI_RSDP_XCHECKSUM_LENGTH)))
+			continue;
+
+		/* Signature and checksum valid, we have found a real RSDP */
+		return address;
+	}
+	return NULL;
+}
+
+/* Search RSDP address, based on acpi_find_root_pointer(). */
+static acpi_physical_address bios_get_rsdp_addr(void)
+{
+	u8 *table_ptr;
+	u32 address;
+	u8 *rsdp;
+
+	/* Get the location of the Extended BIOS Data Area (EBDA) */
+	table_ptr = (u8 *)ACPI_EBDA_PTR_LOCATION;
+	address = *(u16 *)table_ptr;
+	address <<= 4;
+	table_ptr = (u8 *)(long)address;
+
+	/*
+	 * Search EBDA paragraphs (EBDA is required to be a minimum of
+	 * 1K length)
+	 */
+	if (address > 0x400) {
+		rsdp = scan_mem_for_rsdp(table_ptr, ACPI_EBDA_WINDOW_SIZE);
+		if (rsdp) {
+			address += (u32)ACPI_PTR_DIFF(rsdp, table_ptr);
+			return address;
+		}
+	}
+
+	/* Search upper memory: 16-byte boundaries in E0000h-FFFFFh */
+	table_ptr = (u8 *)ACPI_HI_RSDP_WINDOW_BASE;
+	rsdp = scan_mem_for_rsdp(table_ptr, ACPI_HI_RSDP_WINDOW_SIZE);
+
+	if (rsdp) {
+		address = (u32)(ACPI_HI_RSDP_WINDOW_BASE +
+				ACPI_PTR_DIFF(rsdp, table_ptr));
+		return address;
+	}
+	return 0;
+}
-- 
2.20.1