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

[Tom Lendacky <thomas.lendacky@amd.com>]

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.

Reported-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/kernel/mpparse.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/mpparse.c b/arch/x86/kernel/mpparse.c
index 5cbb317..c3d3094 100644
--- a/arch/x86/kernel/mpparse.c
+++ b/arch/x86/kernel/mpparse.c
@@ -430,6 +430,7 @@ 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)
 {
@@ -503,7 +504,7 @@ 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)
@@ -592,6 +593,7 @@ 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);
@@ -857,7 +859,7 @@ 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));