Re: [PATCHv2 2/5] x86/boot/compressed/64: Find a place for 32-bit trampoline

If a bootloader enables 64-bit mode with 4-level paging, we might need to
switch over to 5-level paging. The switching requires the disabling of
paging, which works fine if kernel itself is loaded below 4G.

But if the bootloader puts the kernel above 4G (not sure if anybody does
this), we would lose control as soon as paging is disabled, because the
code becomes unreachable to the CPU.

To handle the situation, we need a trampoline in lower memory that would
take care of switching on 5-level paging.

This patch finds a spot in low memory for a trampoline.

The heuristic is based on code in reserve_bios_regions().

We find the end of low memory based on BIOS and EBDA start addresses.
The trampoline is put just before end of low memory. It's mimic approach
taken to allocate memory for realtime trampoline.

Signed-off-by: Kirill A. Shutemov <redacted>
Tested-by: Borislav Petkov <redacted>
---
 arch/x86/boot/compressed/misc.c       |  4 ++++
 arch/x86/boot/compressed/pgtable.h    | 11 +++++++++++
 arch/x86/boot/compressed/pgtable_64.c | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+)
 create mode 100644 arch/x86/boot/compressed/pgtable.h

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b50c42455e25..e58409667b13 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 +373,9 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	debug_putaddr(output_len);
 	debug_putaddr(kernel_total_size);
 
+	/* Report address of 32-bit trampoline */
+	debug_putaddr(trampoline_32bit);
+
 	/*
 	 * The memory hole needed for the kernel is the larger of either
 	 * the entire decompressed kernel plus relocation table, or the