Re: [PATCHv2 0/4] x86: 5-level related changes into decompression code

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Wed, Nov 29, 2017 at 05:48:51PM +0000, Borislav Petkov wrote:
> On Wed, Nov 29, 2017 at 08:08:31PM +0300, Kirill A. Shutemov wrote:
> > We're really early in the boot -- startup_64 in decompression code -- and
> > I don't know a way print a message there. Is there a way?
> > 
> > no_longmode handled by just hanging the machine. Is it enough for no_la57
> > case too?
> 
> Patch pls.

The patch below on top of patch 2/4 from this patch would do the trick.

Please give it a shot.

>From 95b5489d1f4ea03c6226d13eb6797825234489d6 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Thu, 30 Nov 2017 10:23:53 +0300
Subject: [PATCH] x86/boot/compressed/64: Print error if 5-level paging is not
 supported

We cannot proceed booting if the machine doesn't support the paging mode
kernel was compiled for.

Getting error the usual way -- via validate_cpu() -- is not going to
work. We need to enable appropriate paging mode before that, otherwise
kernel would triple-fault during KASLR setup.

This code will go away once we get support for boot-time switching
between paging modes.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/boot/compressed/misc.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b50c42455e25..5205e848dc33 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -40,6 +40,8 @@
 /* Functions used by the included decompressor code below. */
 void *memmove(void *dest, const void *src, size_t n);
 
+int l5_paging_required(void);
+
 /*
  * This is set up by the setup-routine at boot-time
  */
@@ -362,6 +364,13 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	console_init();
 	debug_putstr("early console in extract_kernel\n");
 
+	if (IS_ENABLED(CONFIG_X86_5LEVEL) && !l5_paging_required()) {
+		error("The kernel is compiled with 5-level paging enabled, "
+				"but the CPU doesn't support la57\n"
+				"Unable to boot - please use "
+				"a kernel appropriate for your CPU.\n");
+	}
+
 	free_mem_ptr     = heap;	/* Heap */
 	free_mem_end_ptr = heap + BOOT_HEAP_SIZE;
 
-- 
 Kirill A. Shutemov