Gcc12 takes issue with core_parking_remove()'s

    for ( ; i < cur_idle_nums; ++i )
        core_parking_cpunum[i] = core_parking_cpunum[i + 1];

complaining that the right hand side array access is past the bounds of
1. Clearly the compiler can't know that cur_idle_nums can only ever be
zero in this case (as the sole CPU cannot be parked).

Arrange for core_parking.c's contents to not be needed altogether, and
then disable its building when NR_CPUS == 1.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v4: Restrict the arch_do_sysctl() change.
v3: Use "imply" and "depends on" in Kconfig. Adjust the arch_do_sysctl()
    change.
v2: Disable building of core_parking.c altogether.

--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -10,7 +10,7 @@ config X86
 	select ALTERNATIVE_CALL
 	select ARCH_MAP_DOMAIN_PAGE
 	select ARCH_SUPPORTS_INT128
-	select CORE_PARKING
+	imply CORE_PARKING
 	select HAS_ALTERNATIVE
 	select HAS_COMPAT
 	select HAS_CPUFREQ
--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -727,12 +727,17 @@ ret_t do_platform_op(
         case XEN_CORE_PARKING_SET:
             idle_nums = min_t(uint32_t,
                     op->u.core_parking.idle_nums, num_present_cpus() - 1);
-            ret = continue_hypercall_on_cpu(
-                    0, core_parking_helper, (void *)(unsigned long)idle_nums);
+            if ( CONFIG_NR_CPUS > 1 )
+                ret = continue_hypercall_on_cpu(
+                        0, core_parking_helper,
+                        (void *)(unsigned long)idle_nums);
+            else if ( idle_nums )
+                ret = -EINVAL;
             break;
 
         case XEN_CORE_PARKING_GET:
-            op->u.core_parking.idle_nums = get_cur_idle_nums();
+            op->u.core_parking.idle_nums = CONFIG_NR_CPUS > 1
+                                           ? get_cur_idle_nums() : 0;
             ret = __copy_field_to_guest(u_xenpf_op, op, u.core_parking) ?
                   -EFAULT : 0;
             break;
--- a/xen/arch/x86/sysctl.c
+++ b/xen/arch/x86/sysctl.c
@@ -178,6 +178,9 @@ long arch_do_sysctl(
                 ret = -EBUSY;
                 break;
             }
+            if ( CONFIG_NR_CPUS <= 1 )
+                /* Mimic behavior of smt_up_down_helper(). */
+                return 0;
             plug = op == XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE;
             fn = smt_up_down_helper;
             hcpu = _p(plug);
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -10,6 +10,7 @@ config COMPAT
 
 config CORE_PARKING
 	bool
+	depends on NR_CPUS > 1
 
 config GRANT_TABLE
 	bool "Grant table support" if EXPERT

On 28/02/2023 12:34 pm, Jan Beulich wrote:
> Gcc12 takes issue with core_parking_remove()'s
>
>     for ( ; i < cur_idle_nums; ++i )
>         core_parking_cpunum[i] = core_parking_cpunum[i + 1];
>
> complaining that the right hand side array access is past the bounds of
> 1. Clearly the compiler can't know that cur_idle_nums can only ever be
> zero in this case (as the sole CPU cannot be parked).
>
> Arrange for core_parking.c's contents to not be needed altogether, and
> then disable its building when NR_CPUS == 1.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>