3 files changed, 51 insertions, 0 deletions

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b50c42455e25..8e4b55dd5df9 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -14,6 +14,7 @@
 
 #include "misc.h"
 #include "error.h"
+#include "pgtable.h"
 #include "../string.h"
 #include "../voffset.h"
 
@@ -372,6 +373,11 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
         debug_putaddr(output_len);
         debug_putaddr(kernel_total_size);
 
+#ifdef CONFIG_X86_64
+        /* Report address of 32-bit trampoline */
+        debug_putaddr(trampoline_32bit);
+#endif
+
         /*
          * The memory hole needed for the kernel is the larger of either
          * the entire decompressed kernel plus relocation table, or the
diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
new file mode 100644
index 000000000000..57722a2fe2a0
--- /dev/null
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -0,0 +1,11 @@
+#ifndef BOOT_COMPRESSED_PAGETABLE_H
+#define BOOT_COMPRESSED_PAGETABLE_H
+
+#define TRAMPOLINE_32BIT_SIZE           (2 * PAGE_SIZE)
+
+#ifndef __ASSEMBLER__
+
+extern unsigned long *trampoline_32bit;
+
+#endif /* __ASSEMBLER__ */
+#endif /* BOOT_COMPRESSED_PAGETABLE_H */
diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 45c76eff2718..21d5cc1cd5fa 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -1,4 +1,5 @@
 #include <asm/processor.h>
+#include "pgtable.h"
 
 /*
  * __force_order is used by special_insns.h asm code to force instruction
@@ -9,14 +10,27 @@
  */
 unsigned long __force_order;
 
+#define BIOS_START_MIN          0x20000U        /* 128K, less than this is insane */
+#define BIOS_START_MAX          0x9f000U        /* 640K, absolute maximum */
+
 struct paging_config {
         unsigned long trampoline_start;
         unsigned long l5_required;
 };
 
+/*
+ * Trampoline address will be printed by extract_kernel() for debugging
+ * purposes.
+ *
+ * Avoid putting the pointer into .bss as it will be cleared between
+ * paging_prepare() and extract_kernel().
+ */
+unsigned long *trampoline_32bit __section(.data);
+
 struct paging_config paging_prepare(void)
 {
         struct paging_config paging_config = {};
+        unsigned long bios_start, ebda_start;
 
         /*
          * Check if LA57 is desired and supported.
@@ -35,5 +49,25 @@ struct paging_config paging_prepare(void)
                 paging_config.l5_required = 1;
         }
 
+        /*
+         * Find a suitable spot for the trampoline.
+         * This code is based on reserve_bios_regions().
+         */
+
+        ebda_start = *(unsigned short *)0x40e << 4;
+        bios_start = *(unsigned short *)0x413 << 10;
+
+        if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
+                bios_start = BIOS_START_MAX;
+
+        if (ebda_start > BIOS_START_MIN && ebda_start < bios_start)
+                bios_start = ebda_start;
+
+        /* Place the trampoline just below the end of low memory, aligned to 4k */
+        paging_config.trampoline_start = bios_start - TRAMPOLINE_32BIT_SIZE;
+        paging_config.trampoline_start = round_down(paging_config.trampoline_start, PAGE_SIZE);
+
+        trampoline_32bit = (unsigned long *)paging_config.trampoline_start;
+
         return paging_config;
 }