[sw-sysemu] Fix rvtimer write_mtimecmp immediate interrupt raise

sw-sysemu/devices/rvtimer.h

@@ -57,6 +57,13 @@ struct RVTimer
                     agent.chip->clear_machine_timer_interrupt(shire);
                 }
             }
+        } else if (!had_interrupt && interrupt) {
+            // raise immediately if mtimecmp is written in the past.
+            for (unsigned shire = 0; shire < EMU_NUM_SHIRES; ++shire) {
+                if ((interrupt_shire_mask >> shire) & 1) {
+                    agent.chip->raise_machine_timer_interrupt(shire);
+                }
+            }
         }
     }
 

sw-sysemu/tests/erbium/src/rvtimer_mtimecmp_past.c

@@ -0,0 +1,53 @@
+/*-------------------------------------------------------------------------
+* Copyright (c) 2026 Ainekko, Co.
+* SPDX-License-Identifier: Apache-2.0
+*-------------------------------------------------------------------------*/
+
+/*
+* Test: MTIMECMP written "in the past" immediately asserts MTIP.
+* Expected: PASS (mip.MTIP set immediately after write)
+*/
+
+#include "test.h"
+#include <stdint.h>
+
+#define ESR_MTIME              0x80F40200ull
+#define ESR_MTIMECMP           0x80F40208ull
+#define ESR_MTIME_LOCAL_TARGET 0x80F40218ull
+
+#define MIP_MTIP (1UL << 7)
+
+static inline uint64_t read_mip(void)
+{
+    uint64_t val;
+    asm volatile("csrr %0, mip" : "=r"(val));
+    return val;
+}
+
+int main(void)
+{
+    volatile uint64_t *mtime = (volatile uint64_t *)ESR_MTIME;
+    volatile uint64_t *mtimecmp = (volatile uint64_t *)ESR_MTIMECMP;
+    volatile uint64_t *mtime_target = (volatile uint64_t *)ESR_MTIME_LOCAL_TARGET;
+
+    /* Enable timer interrupt delivery for minion 0. */
+    *mtime_target = 0x1;
+
+    /* Start from a known state with no pending timer interrupt. */
+    *mtime = 1000;
+    *mtimecmp = 1000000;
+
+    if (read_mip() & MIP_MTIP) {
+        TEST_FAIL;
+    }
+
+    /* Write MTIMECMP "in the past": should assert MTIP immediately. */
+    *mtimecmp = 500;
+
+    if (read_mip() & MIP_MTIP) {
+        TEST_PASS;
+    }
+
+    TEST_FAIL;
+    return 0;
+}