x86/setup: Move trim_snb_memory() later in setup_arch() to fix boot hangs

Commit

  a799c2bd ("x86/setup: Consolidate early memory reservations")

moved reservation of the memory inaccessible by Sandy Bride integrated
graphics very early, and, as a result, on systems with such devices
the first 1M was reserved by trim_snb_memory() which prevented the
allocation of the real mode trampoline and made the boot hang very
early.

Since the purpose of trim_snb_memory() is to prevent problematic pages
ever reaching the graphics device, it is safe to reserve these pages
after memblock allocations are possible.

Move trim_snb_memory() later in boot so that it will be called after
reserve_real_mode() and make comments describing trim_snb_memory()
operation more elaborate.

 [ bp: Massage a bit. ]

Fixes: a799c2bd ("x86/setup: Consolidate early memory reservations")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Hugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/f67d3e03-af90-f790-baf4-8d412fe055af@infradead.org

arch/x86/kernel/setup.c

@@ -633,11 +633,16 @@ static void __init trim_snb_memory(void)
 	printk(KERN_DEBUG "reserving inaccessible SNB gfx pages\n");
 
 	/*
-	 * Reserve all memory below the 1 MB mark that has not
-	 * already been reserved.
+	 * SandyBridge integrated graphics devices have a bug that prevents
+	 * them from accessing certain memory ranges, namely anything below
+	 * 1M and in the pages listed in bad_pages[] above.
+	 *
+	 * To avoid these pages being ever accessed by SNB gfx devices
+	 * reserve all memory below the 1 MB mark and bad_pages that have
+	 * not already been reserved at boot time.
 	 */
 	memblock_reserve(0, 1<<20);
-	
+
 	for (i = 0; i < ARRAY_SIZE(bad_pages); i++) {
 		if (memblock_reserve(bad_pages[i], PAGE_SIZE))
 			printk(KERN_WARNING "failed to reserve 0x%08lx\n",
@@ -746,8 +751,6 @@ static void __init early_reserve_memory(void)
 
 	reserve_ibft_region();
 	reserve_bios_regions();
-
-	trim_snb_memory();
 }
 
 /*
@@ -1081,6 +1084,13 @@ void __init setup_arch(char **cmdline_p)
 
 	reserve_real_mode();
 
+	/*
+	 * Reserving memory causing GPU hangs on Sandy Bridge integrated
+	 * graphics devices should be done after we allocated memory under
+	 * 1M for the real mode trampoline.
+	 */
+	trim_snb_memory();
+
 	init_mem_mapping();
 
 	idt_setup_early_pf();