x86/boot: Add ACPI RSDP address to setup_header

Xen PVH guests receive the address of the RSDP table from Xen. In order
to support booting a Xen PVH guest via Grub2 using the standard x86
boot entry we need a way for Grub2 to pass the RSDP address to the
kernel.

For this purpose expand the struct setup_header to hold the physical
address of the RSDP address. Being zero means it isn't specified and
has to be located the legacy way (searching through low memory or
EBDA).

While documenting the new setup_header layout and protocol version
2.14 add the missing documentation of protocol version 2.13.

There are Grub2 versions in several distros with a downstream patch
violating the boot protocol by writing past the end of setup_header.
This requires another update of the boot protocol to enable the kernel
to distinguish between a specified RSDP address and one filled with
garbage by such a broken Grub2.

From protocol 2.14 on Grub2 will write the version it is supporting
(but never a higher value than found to be supported by the kernel)
ored with 0x8000 to the version field of setup_header. This enables
the kernel to know up to which field Grub2 has written information
to. All fields after that are supposed to be clobbered.

Signed-off-by: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boris.ostrovsky@oracle.com
Cc: bp@alien8.de
Cc: corbet@lwn.net
Cc: linux-doc@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20181010061456.22238-3-jgross@suse.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

6 files changed, 31 insertions, 1 deletions

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e8896..4c881c850125c 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -300,7 +300,7 @@ _start:
 	# Part 2 of the header, from the old setup.S
 
 		.ascii	"HdrS"		# header signature
-		.word	0x020d		# header version number (>= 0x0105)
+		.word	0x020e		# header version number (>= 0x0105)
 					# or else old loadlin-1.5 will fail)
 		.globl realmode_swtch
 realmode_swtch:	.word	0, 0		# default_switch, SETUPSEG
@@ -558,6 +558,10 @@ pref_address:		.quad LOAD_PHYSICAL_ADDR	# preferred load addr
 init_size:		.long INIT_SIZE		# kernel initialization size
 handover_offset:	.long 0			# Filled in by build.c
 
+acpi_rsdp_addr:		.quad 0			# 64-bit physical pointer to the
+						# ACPI RSDP table, added with
+						# version 2.14
+
 # End of setup header #####################################################
 
 	.section ".entrytext", "ax"
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index b85a7c54c6a13..0f842104862c3 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -303,4 +303,6 @@ extern void x86_init_noop(void);
 extern void x86_init_uint_noop(unsigned int unused);
 extern bool x86_pnpbios_disabled(void);
 
+void x86_verify_bootdata_version(void);
+
 #endif
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index a06cbf0197446..22f89d040dddc 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -16,6 +16,9 @@
 #define RAMDISK_PROMPT_FLAG		0x8000
 #define RAMDISK_LOAD_FLAG		0x4000
 
+/* version flags */
+#define VERSION_WRITTEN	0x8000
+
 /* loadflags */
 #define LOADED_HIGH	(1<<0)
 #define KASLR_FLAG	(1<<1)
@@ -86,6 +89,7 @@ struct setup_header {
 	__u64	pref_address;
 	__u32	init_size;
 	__u32	handover_offset;
+	__u64	acpi_rsdp_addr;
 } __attribute__((packed));
 
 struct sys_desc_table {
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index ec6fefbfd3c04..76fa3b8365985 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -37,6 +37,7 @@ asmlinkage __visible void __init i386_start_kernel(void)
 	cr4_init_shadow();
 
 	sanitize_boot_params(&boot_params);
+	x86_verify_bootdata_version();
 
 	x86_early_init_platform_quirks();
 
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index ddee1f0870c4b..5dc377dc9d7b5 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -457,6 +457,8 @@ void __init x86_64_start_reservations(char *real_mode_data)
 	if (!boot_params.hdr.version)
 		copy_bootdata(__va(real_mode_data));
 
+	x86_verify_bootdata_version();
+
 	x86_early_init_platform_quirks();
 
 	switch (boot_params.hdr.hardware_subarch) {
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index b4866badb235a..8e226b7a17534 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1281,6 +1281,23 @@ void __init setup_arch(char **cmdline_p)
 	unwind_init();
 }
 
+/*
+ * From boot protocol 2.14 onwards we expect the bootloader to set the
+ * version to "0x8000 | <used version>". In case we find a version >= 2.14
+ * without the 0x8000 we assume the boot loader supports 2.13 only and
+ * reset the version accordingly. The 0x8000 flag is removed in any case.
+ */
+void __init x86_verify_bootdata_version(void)
+{
+	if (boot_params.hdr.version & VERSION_WRITTEN)
+		boot_params.hdr.version &= ~VERSION_WRITTEN;
+	else if (boot_params.hdr.version >= 0x020e)
+		boot_params.hdr.version = 0x020d;
+
+	if (boot_params.hdr.version < 0x020e)
+		boot_params.hdr.acpi_rsdp_addr = 0;
+}
+
 #ifdef CONFIG_X86_32
 
 static struct resource video_ram_resource = {