[PATCH v5 3/3] xen/arm: mpu: Implement a dummy enable_secondary_cpu_mm

[Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>]

Secondary cpus initialization is not yet supported. Thus, we print an
appropriate message and put the secondary cpus in WFE state.
And we introduce to BUILD_BUG_ON to prevent users using from building Xen
on multiprocessor based MPU systems.

In Arm, there is no clean way to disable SMP. As of now, we wish to support
MPU on UNP only. So, we have defined the default range of NR_CPUs to be 1 for
MPU.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
---
Changes from :-

v1 - 1. NR_CPUS is defined as 1 for MPU

2. Added a message in enable_secondary_cpu_mm()

v2 - 1. Added the range

2. Clarified in the commit message why/how we have disabled SMP.

v3 - 1. BUILD_BUG_ON() is moved to smp.c.

v4 - 1. Moved "default "1" if ARM && MPU” right after “default "256" if X86”.

 xen/arch/Kconfig              |  2 ++
 xen/arch/arm/arm64/mpu/head.S | 10 ++++++++++
 xen/arch/arm/smp.c            | 11 +++++++++++
 3 files changed, 23 insertions(+)

diff --git a/xen/arch/Kconfig b/xen/arch/Kconfig
index 308ce129a8..9f4835e37f 100644
--- a/xen/arch/Kconfig
+++ b/xen/arch/Kconfig
@@ -6,8 +6,10 @@ config PHYS_ADDR_T_32
 
 config NR_CPUS
        int "Maximum number of CPUs"
+       range 1 1 if ARM && MPU
        range 1 16383
        default "256" if X86
+       default "1" if ARM && MPU
        default "8" if ARM && RCAR3
        default "4" if ARM && QEMU
        default "4" if ARM && MPSOC
diff --git a/xen/arch/arm/arm64/mpu/head.S b/xen/arch/arm/arm64/mpu/head.S
index a449aeca67..731698aa3b 100644
--- a/xen/arch/arm/arm64/mpu/head.S
+++ b/xen/arch/arm/arm64/mpu/head.S
@@ -141,6 +141,16 @@ FUNC(enable_boot_cpu_mm)
     ret
 END(enable_boot_cpu_mm)
 
+/*
+ * We don't yet support secondary CPUs bring-up. Implement a dummy helper to
+ * please the common code.
+ */
+ENTRY(enable_secondary_cpu_mm)
+    PRINT("- SMP not enabled yet -\r\n")
+1:  wfe
+    b 1b
+ENDPROC(enable_secondary_cpu_mm)
+
 /*
  * Local variables:
  * mode: ASM
diff --git a/xen/arch/arm/smp.c b/xen/arch/arm/smp.c
index c11bba93ad..b372472188 100644
--- a/xen/arch/arm/smp.c
+++ b/xen/arch/arm/smp.c
@@ -1,4 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
+#include <xen/init.h>
 #include <xen/mm.h>
 #include <asm/system.h>
 #include <asm/smp.h>
@@ -6,6 +7,16 @@
 #include <asm/gic.h>
 #include <asm/flushtlb.h>
 
+static void __init __maybe_unused build_assertions(void)
+{
+#ifdef CONFIG_MPU
+    /*
+     * Currently, SMP is not enabled on MPU based systems.
+     */
+    BUILD_BUG_ON(NR_CPUS > 1);
+#endif
+}
+
 void arch_flush_tlb_mask(const cpumask_t *mask)
 {
     /* No need to IPI other processors on ARM, the processor takes care of it. 
*/
-- 
2.25.1