x86/boot: Simplify EBDA-vs-BIOS reservation logic

Both the intent and the effect of reserve_bios_regions() is simple:
reserve the range from the apparent BIOS start (suitably filtered)
through 1MB and, if the EBDA start address is sensible, extend that
reservation downward to cover the EBDA as well.

The code is overcomplicated, though, and contains head-scratchers
like:

	if (ebda_start < BIOS_START_MIN)
		ebda_start = BIOS_START_MAX;

That snipped is trying to say "if ebda_start < BIOS_START_MIN,
ignore it".

Simplify it: reorder the code so that it makes sense.  This should
have no functional effect under any circumstances.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Link: http://lkml.kernel.org/r/ef89c0c761be20ead8bd9a3275743e6259b6092a.1469135598.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/ebda.c

@@ -65,22 +65,6 @@ void __init reserve_bios_regions(void)
 	if (!x86_platform.legacy.reserve_bios_regions)
 		return;
 
-	/* Get the start address of the EBDA page: */
-	ebda_start = get_bios_ebda();
-
-	/*
-	 * Quirk: some old Dells seem to have a 4k EBDA without
-	 * reporting so in their BIOS RAM size value, so just
-	 * consider the memory above 640K to be off limits
-	 * (bugzilla 2990).
-	 *
-	 * We detect this case by filtering for nonsensical EBDA
-	 * addresses below 128K, where we can assume that they
-	 * are bogus and bump it up to a fixed 640K value:
-	 */
-	if (ebda_start < BIOS_START_MIN)
-		ebda_start = BIOS_START_MAX;
-
 	/*
 	 * BIOS RAM size is encoded in kilobytes, convert it
 	 * to bytes to get a first guess at where the BIOS
@@ -91,18 +75,22 @@ void __init reserve_bios_regions(void)
 
 	/*
 	 * If bios_start is less than 128K, assume it is bogus
-	 * and bump it up to 640K:
+	 * and bump it up to 640K.  Similarly, if bios_start is above 640K,
+	 * don't trust it.
 	 */
-	if (bios_start < BIOS_START_MIN)
+	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
 		bios_start = BIOS_START_MAX;
 
+	/* Get the start address of the EBDA page: */
+	ebda_start = get_bios_ebda();
+
 	/*
-	 * Use the lower of the bios_start and ebda_start
-	 * as the starting point, but don't allow it to
-	 * go beyond 640K:
+	 * If the EBDA start address is sane and is below the BIOS region,
+	 * then also reserve everything from the EBDA start address up to
+	 * the BIOS region.
 	 */
-	bios_start = min(bios_start, ebda_start);
-	bios_start = min(bios_start, BIOS_START_MAX);
+	if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
+		bios_start = ebda_start;
 
 	/* Reserve all memory between bios_start and the 1MB mark: */
 	memblock_reserve(bios_start, 0x100000 - bios_start);