x86, reboot: Move the real-mode reboot code to an assembly file

Move the real-mode reboot code out to an assembly file (reboot_32.S)
which is allocated using the common lowmem trampoline allocator.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
LKML-Reference: <4D5DFBE4.7090104@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Matthieu Castet <castet.matthieu@free.fr>

1 files changed, 131 insertions, 0 deletions

diff --git a/arch/x86/kernel/reboot_32.S b/arch/x86/kernel/reboot_32.S
new file mode 100644
index 00000000000..f242356a096
--- /dev/null
+++ b/arch/x86/kernel/reboot_32.S
@@ -0,0 +1,131 @@
+#include <linux/linkage.h>
+#include <linux/init.h>
+#include <asm/segment.h>
+#include <asm/page_types.h>
+
+/*
+ * The following code and data reboots the machine by switching to real
+ * mode and jumping to the BIOS reset entry point, as if the CPU has
+ * really been reset.  The previous version asked the keyboard
+ * controller to pulse the CPU reset line, which is more thorough, but
+ * doesn't work with at least one type of 486 motherboard.  It is easy
+ * to stop this code working; hence the copious comments.
+ *
+ * This code is called with the restart type (0 = BIOS, 1 = APM) in %eax.
+ */
+        .section ".x86_trampoline","a"
+        .balign 16
+        .code32
+ENTRY(machine_real_restart_asm)
+r_base = .
+        /* Get our own relocated address */
+        call    1f
+1:      popl    %ebx
+        subl    $1b, %ebx
+
+        /* Patch post-real-mode segment jump */
+        movw    dispatch_table(%ebx,%ecx,2),%cx
+        movw    %cx, 101f(%ebx)
+        movw    %ax, 102f(%ebx)
+
+        /* Set up the IDT for real mode. */
+        lidtl   machine_real_restart_idt(%ebx)
+
+        /*
+         * Set up a GDT from which we can load segment descriptors for real
+         * mode.  The GDT is not used in real mode; it is just needed here to
+         * prepare the descriptors.
+         */
+        lgdtl   machine_real_restart_gdt(%ebx)
+
+        /*
+         * Load the data segment registers with 16-bit compatible values
+         */
+        movl    $16, %ecx
+        movl    %ecx, %ds
+        movl    %ecx, %es
+        movl    %ecx, %fs
+        movl    %ecx, %gs
+        movl    %ecx, %ss
+        ljmpl   $8, $1f - r_base
+
+/*
+ * This is 16-bit protected mode code to disable paging and the cache,
+ * switch to real mode and jump to the BIOS reset code.
+ *
+ * The instruction that switches to real mode by writing to CR0 must be
+ * followed immediately by a far jump instruction, which set CS to a
+ * valid value for real mode, and flushes the prefetch queue to avoid
+ * running instructions that have already been decoded in protected
+ * mode.
+ *
+ * Clears all the flags except ET, especially PG (paging), PE
+ * (protected-mode enable) and TS (task switch for coprocessor state
+ * save).  Flushes the TLB after paging has been disabled.  Sets CD and
+ * NW, to disable the cache on a 486, and invalidates the cache.  This
+ * is more like the state of a 486 after reset.  I don't know if
+ * something else should be done for other chips.
+ *
+ * More could be done here to set up the registers as if a CPU reset had
+ * occurred; hopefully real BIOSs don't assume much.  This is not the
+ * actual BIOS entry point, anyway (that is at 0xfffffff0).
+ *
+ * Most of this work is probably excessive, but it is what is tested.
+ */
+        .code16
+1:
+        xorl    %ecx, %ecx
+        movl    %cr0, %eax
+        andl    $0x00000011, %eax
+        orl     $0x60000000, %eax
+        movl    %eax, %cr0
+        movl    %ecx, %cr3
+        movl    %cr0, %edx
+        andl    $0x60000000, %edx       /* If no cache bits -> no wbinvd */
+        jz      2f
+        wbinvd
+2:
+        andb    $0x10, %al
+        movl    %eax, %cr0
+        .byte   0xea                    /* ljmpw */
+101:    .word   0                       /* Offset */
+102:    .word   0                       /* Segment */
+
+bios:
+        ljmpw   $0xf000, $0xfff0
+
+apm:
+        movw    $0x1000, %ax
+        movw    %ax, %ss
+        movw    $0xf000, %sp
+        movw    $0x5307, %ax
+        movw    $0x0001, %bx
+        movw    $0x0003, %cx
+        int     $0x15
+
+END(machine_real_restart_asm)
+
+        .balign 16
+        /* These must match <asm/reboot.h */
+dispatch_table:
+        .word   bios - r_base
+        .word   apm - r_base
+END(dispatch_table)
+
+        .balign 16
+machine_real_restart_idt:
+        .word   0xffff          /* Length - real mode default value */
+        .long   0               /* Base - real mode default value */
+END(machine_real_restart_idt)
+
+        .balign 16
+ENTRY(machine_real_restart_gdt)
+        .quad   0               /* Self-pointer, filled in by PM code */
+        .quad   0               /* 16-bit code segment, filled in by PM code */
+        /*
+         * 16-bit data segment with the selector value 16 = 0x10 and
+         * base value 0x100; since this is consistent with real mode
+         * semantics we don't have to reload the segments once CR0.PE = 0.
+         */
+        .quad   GDT_ENTRY(0x0093, 0x100, 0xffff)
+END(machine_real_restart_gdt)