RISC-V SMP support is not yet implemented, but smp_send_event_check_mask()
is required by common code and vcpu_kick(), which is introduced later.
Provide a temporary stub implementation that asserts the mask only targets
CPU0.

cpumask_subset() is used instead of cpumask_equal() because some callers
(e.g. cpumask_raise_softirq() or cpu_raise_softirq_batch_finish()) may
legitimately pass an empty mask, which would otherwise cause false
failures.

The BUG_ON() ensures that attempts to use this function with multiple CPUs
are caught early once SMP support is introduced.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes in v2:
 - use BUG_ON(cpumask_subset(...)) instead of "#ifdef NR_CPUS > 1".
 - Update the commit message.
---
 xen/arch/riscv/smp.c   | 7 +++++++
 xen/arch/riscv/stubs.c | 5 -----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/xen/arch/riscv/smp.c b/xen/arch/riscv/smp.c
index 4ca6a4e89200..d645364ea47d 100644
--- a/xen/arch/riscv/smp.c
+++ b/xen/arch/riscv/smp.c
@@ -1,3 +1,4 @@
+#include <xen/cpumask.h>
 #include <xen/smp.h>
 
 /*
@@ -13,3 +14,9 @@
 struct pcpu_info pcpu_info[NR_CPUS] = { [0 ... NR_CPUS - 1] = {
     .processor_id = NR_CPUS,
 }};
+
+void smp_send_event_check_mask(const cpumask_t *mask)
+{
+    /* Catch missing implementation once SMP support is introduced */
+    BUG_ON(!cpumask_subset(mask, cpumask_of(0)));
+}
diff --git a/xen/arch/riscv/stubs.c b/xen/arch/riscv/stubs.c
index 9e30a9a3b50b..c5784a436574 100644
--- a/xen/arch/riscv/stubs.c
+++ b/xen/arch/riscv/stubs.c
@@ -65,11 +65,6 @@ int arch_monitor_domctl_event(struct domain *d,
 
 /* smp.c */
 
-void smp_send_event_check_mask(const cpumask_t *mask)
-{
-    BUG_ON("unimplemented");
-}
-
 void smp_send_call_function_mask(const cpumask_t *mask)
 {
     BUG_ON("unimplemented");
-- 
2.52.0