x86/boot: Fix boot failure when SMP MP-table is based at 0

When crosvm is used to boot a kernel as a VM, the SMP MP-table is found
at physical address 0x0. This causes mpf_base to be set to 0 and a
subsequent "if (!mpf_base)" check in default_get_smp_config() results in
the MP-table not being parsed.  Further into the boot this results in an
oops when attempting a read_apic_id().

Add a boolean variable that is set to true when the MP-table is found.
Use this variable for testing if the MP-table was found so that even a
value of 0 for mpf_base will result in continued parsing of the MP-table.

Fixes: 5997efb ("x86/boot: Use memremap() to map the MPF and MPC data")
Reported-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: regression@leemhuis.info
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20171106201753.23059.86674.stgit@tlendack-t1.amdoffice.net

arch/x86/kernel/mpparse.c

@@ -431,6 +431,7 @@ static inline void __init construct_default_ISA_mptable(int mpc_default_type)
 }
 
 static unsigned long mpf_base;
+static bool mpf_found;
 
 static unsigned long __init get_mpc_size(unsigned long physptr)
 {
@@ -504,7 +505,7 @@ void __init default_get_smp_config(unsigned int early)
 	if (!smp_found_config)
 		return;
 
-	if (!mpf_base)
+	if (!mpf_found)
 		return;
 
 	if (acpi_lapic && early)
@@ -593,6 +594,7 @@ static int __init smp_scan_config(unsigned long base, unsigned long length)
 			smp_found_config = 1;
 #endif
 			mpf_base = base;
+			mpf_found = true;
 
 			pr_info("found SMP MP-table at [mem %#010lx-%#010lx] mapped at [%p]\n",
 				base, base + sizeof(*mpf) - 1, mpf);
@@ -858,7 +860,7 @@ static int __init update_mp_table(void)
 	if (!enable_update_mptable)
 		return 0;
 
-	if (!mpf_base)
+	if (!mpf_found)
 		return 0;
 
 	mpf = early_memremap(mpf_base, sizeof(*mpf));