RISC-V: implement WFI in the platform model.

The initial implementation tries to optimize for simulator execution, especially for OS boots.

2 files changed, 14 insertions, 2 deletions

diff --git a/riscv/riscv.sail b/riscv/riscv.sail
index c8fcdbab..37553299 100644
--- a/riscv/riscv.sail
+++ b/riscv/riscv.sail
@@ -724,10 +724,10 @@ mapping clause encdec = WFI() <-> 0b000100000101 @ 0b00000 @ 0b000 @ 0b00000 @ 0
 
 function clause execute WFI() =
   match cur_privilege {
-    Machine    => true,
+    Machine    => { platform_wfi(); true },
     Supervisor => if   mstatus.TW() == true
                   then { handle_illegal(); false }
-                  else true,
+                  else { platform_wfi(); true },
     User       => { handle_illegal(); false }
   }
 
diff --git a/riscv/riscv_platform.sail b/riscv/riscv_platform.sail
index c878f2a7..aee72e47 100644
--- a/riscv/riscv_platform.sail
+++ b/riscv/riscv_platform.sail
@@ -281,3 +281,15 @@ function handle_illegal() -> unit = {
                                     excinfo = info };
   nextPC = handle_exception(cur_privilege, CTL_TRAP(t), PC)
 }
+
+/* Platform-specific wait-for-interrupt */
+
+function platform_wfi() -> unit = {
+  /* speed execution by getting the timer to fire at the next instruction,
+   * since we currently don't have any other devices raising interrupts.
+   */
+  if mtime <_u mtimecmp then {
+    mtime  = mtimecmp;
+    mcycle = mtimecmp;
+  }
+}
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