x86/boot: Rework reserve_real_mode() to allow multiple tries

If reserve_real_mode() fails, panicing immediately means we're
doomed. Make it safe to try more than once to allocate the
trampoline:

- Degrade a failure from panic() to pr_info(). (If we make it to
setup_real_mode() without reserving the trampoline, we'll panic
them.)

- Factor out helpers so that platform code can supply a specific
address to try.

- Warn if reserve_real_mode() is called after we're done with the
memblock allocator. If that were to happen, we would behave
unpredictably.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Mario Limonciello <[e-mail]>
Cc: Matt Fleming <[e-mail]>
Cc: Matthew Garrett <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/realmode.h

···
 extern unsigned char secondary_startup_64[];
 #endif
 
+static inline size_t real_mode_size_needed(void)
+{
+	if (real_mode_header)
+		return 0;	/* already allocated. */
+
+	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
+}
+
+void set_real_mode_mem(phys_addr_t mem, size_t size);
 void reserve_real_mode(void);
 
 #endif /* _ARCH_X86_REALMODE_H */

arch/x86/realmode/init.c

···
 #include <linux/io.h>
+#include <linux/slab.h>
 #include <linux/memblock.h>
 
 #include <asm/cacheflush.h>
···
 /* Hold the pgd entry used on booting additional CPUs */
 pgd_t trampoline_pgd_entry;
 
-void __init reserve_real_mode(void)
+void __init set_real_mode_mem(phys_addr_t mem, size_t size)
 {
-	phys_addr_t mem;
-	unsigned char *base;
-	size_t size = PAGE_ALIGN(real_mode_blob_end - real_mode_blob);
+	void *base = __va(mem);
 
-	/* Has to be under 1M so we can execute real-mode AP code. */
-	mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
-	if (!mem)
-		panic("Cannot allocate trampoline\n");
-
-	base = __va(mem);
-	memblock_reserve(mem, size);
 	real_mode_header = (struct real_mode_header *) base;
 	printk(KERN_DEBUG "Base memory trampoline at [%p] %llx size %zu\n",
 	       base, (unsigned long long)mem, size);
+}
+
+void __init reserve_real_mode(void)
+{
+	phys_addr_t mem;
+	size_t size = real_mode_size_needed();
+
+	if (!size)
+		return;
+
+	WARN_ON(slab_is_available());
+
+	/* Has to be under 1M so we can execute real-mode AP code. */
+	mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
+	if (!mem) {
+		pr_info("No sub-1M memory is available for the trampoline\n");
+		return;
+	}
+
+	memblock_reserve(mem, size);
+	set_real_mode_mem(mem, size);
 }
 
 static void __init setup_real_mode(void)