[PATCH v4 2/3] x86/boot: add acpi rsdp address to setup_header

Juergen Gross
2018-10-09T10:54:48Z

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>
---
V3: fix commit message and some documentation bits (Ingo Molnar)

V4: add version feedback from bootloader
---
Documentation/x86/boot.txt | 32 +++++++++++++++++++++++++++++++-
arch/x86/boot/header.S | 6 +++++-
arch/x86/include/asm/x86_init.h | 2 ++
arch/x86/include/uapi/asm/bootparam.h | 4 ++++
arch/x86/kernel/head32.c | 1 +
arch/x86/kernel/head64.c | 2 ++
arch/x86/kernel/setup.c | 17 +++++++++++++++++
7 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
index 5e9b826b5f62..c8f8d7e7a60c 100644
--- a/Documentation/x86/boot.txt
+++ b/Documentation/x86/boot.txt
@@ -61,6 +61,18 @@ Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
to struct boot_params for loading bzImage and ramdisk
above 4G in 64bit.

+Protocol 2.13: (Kernel 3.14) Support 32- and 64-bit flags being set in
+ xloadflags to support booting a 64-bit kernel from 32-bit
+ EFI
+
+Protocol 2.14: (Kernel 4.20) Added acpi_rsdp_addr holding the physical
+ address of the ACPI RSDP table.
+ The bootloader updates version with:
+ 0x8000 | min(kernel-version, bootloader-version)
+ kernel-version being the protocol version supported by
+ the kernel and bootloader-version the protocol version
+ supported by the bootloader.
+
**** MEMORY LAYOUT

The traditional memory map for the kernel loader, used for Image or
@@ -197,6 +209,7 @@ Offset Proto Name Meaning
0258/8 2.10+ pref_address Preferred loading address
0260/4 2.10+ init_size Linear memory required during initialization
0264/4 2.11+ handover_offset Offset of handover entry point
+0268/8 2.14+ acpi_rsdp_addr Physical address of RSDP table

(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
@@ -309,7 +322,7 @@ Protocol: 2.00+
Contains the magic number "HdrS" (0x53726448).

Field name: version
-Type: read
+Type: modify
Offset/size: 0x206/2
Protocol: 2.00+

@@ -317,6 +330,12 @@ Protocol: 2.00+
e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
10.17.

+ Up to protocol version 2.13 this information is only read by the
+ bootloader. From protocol version 2.14 onwards the bootloader will
+ write the used protocol version ored with 0x8000 to the field. The
+ used protocol version will be the minimum of the supported protocol
+ versions of the bootloader and the kernel.
+
Field name: realmode_swtch
Type: modify (optional)
Offset/size: 0x208/4
@@ -744,6 +763,17 @@ Offset/size: 0x264/4

See EFI HANDOVER PROTOCOL below for more details.

+Field name: acpi_rsdp_addr
+Type: write
+Offset/size: 0x268/8
+Protocol: 2.14+
+
+ This field can be set by the boot loader to tell the kernel the
+ physical address of the ACPI RSDP table.
+
+ A value of 0 indicates the kernel should fall back to the standard
+ methods to locate the RSDP.
+

**** THE IMAGE CHECKSUM

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e889..4c881c850125 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 b85a7c54c6a1..0f842104862c 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 a06cbf019744..22f89d040ddd 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 ec6fefbfd3c0..76fa3b836598 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 ddee1f0870c4..5dc377dc9d7b 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 b4866badb235..20321710efb4 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 = {
--
2.16.4