The scheduler may call this function to force synchronization of given
vCPU's state. Although RISC-V does not support lazy context switching,
a full memory barrier is still required to order observation of the
saved context correctly.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes in v2:
 - New patch.
---
 xen/arch/riscv/domain.c | 18 ++++++++++++++++++
 xen/arch/riscv/stubs.c  |  5 -----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/xen/arch/riscv/domain.c b/xen/arch/riscv/domain.c
index 1458902aff82..48ba7584acaa 100644
--- a/xen/arch/riscv/domain.c
+++ b/xen/arch/riscv/domain.c
@@ -259,3 +259,21 @@ void sync_local_execstate(void)
 {
     /* Nothing to do -- no lazy switching */
 }
+
+void sync_vcpu_execstate(struct vcpu *v)
+{
+    /*
+     * We don't support lazy switching.
+     *
+     * However the context may have been saved from a remote pCPU so we
+     * need a barrier to ensure it is observed before continuing.
+     *
+     * Per vcpu_context_saved(), the context can be observed when
+     * v->is_running is false (the caller should check it before calling
+     * this function).
+     *
+     * Note this is a full barrier to also prevent update of the context
+     * to happen before it was observed.
+     */
+    smp_mb();
+}
diff --git a/xen/arch/riscv/stubs.c b/xen/arch/riscv/stubs.c
index c912d46f1e42..26434166acc6 100644
--- a/xen/arch/riscv/stubs.c
+++ b/xen/arch/riscv/stubs.c
@@ -91,11 +91,6 @@ void continue_running(struct vcpu *same)
     BUG_ON("unimplemented");
 }
 
-void sync_vcpu_execstate(struct vcpu *v)
-{
-    BUG_ON("unimplemented");
-}
-
 void startup_cpu_idle_loop(void)
 {
     BUG_ON("unimplemented");
-- 
2.52.0