[PATCH v2 28/39] xen/riscv: handle the case when no vCPU migration is needed

The IMSIC vsfile mapping is performed in continue_new_vcpu(), since the
target pCPU must be known at that point. It is therefore possible for
imsic_migrate_vcpu() to be called before continue_new_vcpu() has
executed, in which case v->arch.last_pcpu is NR_CPUS and there is nothing
to migrate.

Add a BUG_ON("unimplemented") placeholder in imsic_migrate_vcpu() to guard
against silent incorrect behaviour or unexpected panics in guest VMs until
the function is fully implemented.

Signed-off-by: Oleksii Kurochko <[email protected]>
---
Changes in v2:
 - New patch.
---
---
 xen/arch/riscv/imsic.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/xen/arch/riscv/imsic.c b/xen/arch/riscv/imsic.c
index b0c4a9e2d728..ad7fbe708bfd 100644
--- a/xen/arch/riscv/imsic.c
+++ b/xen/arch/riscv/imsic.c
@@ -689,5 +689,15 @@ int __init vimsic_make_domu_dt_node(struct kernel_info *kinfo,
 
 void imsic_migrate_vcpu(struct vcpu *v)
 {
+    /*
+     * The scheduler can mark a freshly created vCPU's unit as migrated and
+     * invoke this before the vCPU has ever run (see the migrated branch in
+     * schedule()). No need to do migration for such vCPUs as they aren't fully
+     * initialized (for example, context_switch() will be called after
+     * imsic_migrate_vcpu()).
+     */
+    if ( v->arch.last_cpu == NR_CPUS )
+        return;
+
     BUG_ON("unimplemented");
 }
-- 
2.55.0