[PATCH v3 6/6] 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>
---
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.

 xen/arch/Kconfig              |  2 ++
 xen/arch/arm/arm64/mpu/head.S | 10 ++++++++++
 xen/arch/arm/setup.c          |  5 +++++
 3 files changed, 17 insertions(+)

diff --git a/xen/arch/Kconfig b/xen/arch/Kconfig
index 308ce129a8..6053e048fa 100644
--- a/xen/arch/Kconfig
+++ b/xen/arch/Kconfig
@@ -6,11 +6,13 @@ config PHYS_ADDR_T_32
 
 config NR_CPUS
        int "Maximum number of CPUs"
+       range 1 1 if MPU
        range 1 16383
        default "256" if X86
        default "8" if ARM && RCAR3
        default "4" if ARM && QEMU
        default "4" if ARM && MPSOC
+       default "1" if ARM && MPU
        default "128" if ARM
        help
          Controls the build-time size of various arrays and bitmaps
diff --git a/xen/arch/arm/arm64/mpu/head.S b/xen/arch/arm/arm64/mpu/head.S
index e354f4552b..4d36a8c9bc 100644
--- a/xen/arch/arm/arm64/mpu/head.S
+++ b/xen/arch/arm/arm64/mpu/head.S
@@ -147,6 +147,16 @@ fail:
     b   1b
 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/setup.c b/xen/arch/arm/setup.c
index 0203771164..5a0d343f5b 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -296,6 +296,11 @@ void asmlinkage __init start_xen(unsigned long fdt_paddr)
     int rc, i;
 
 #ifdef CONFIG_MPU
+    /*
+     * Currently, SMP is not enabled on MPU based systems.
+     */
+    BUILD_BUG_ON(NR_CPUS > 1);
+
     /*
      * Unlike MMU, MPU does not use pages for translation. However, we continue
      * to use PAGE_SIZE to denote 4KB. This is so that the existing memory
-- 
2.25.1