On Tue, Feb 27, 2018 at 06:42:14PM +0300, Kirill A. Shutemov wrote:
> 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 <kirill.shute...@linux.intel.com>
> Tested-by: Borislav Petkov <b...@suse.de>
> ---
> 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
0-day found problem with the patch on 32-bit config.
Here's fixup:
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index e58409667b13..8e4b55dd5df9 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -373,8 +373,10 @@ 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
--
Kirill A. Shutemov
