From 5ee9e9cd8c3b60f346bbbf423e8731f723c61590 Mon Sep 17 00:00:00 2001
From: chiangkd <chiangkuandi@gmail.com>
Date: Mon, 3 Jun 2024 01:04:32 +0800
Subject: [PATCH] Implement Real time counter to prevent mtime overflow

Make 64-bit unsigned integer `mtime` with Real time counter (RTC)
to prevent mtime overflow.

Take 16-bit unsigned integer for example:

mtime    mtimecmp   delta

0xFFF8    0x0004    0x000c
0xFFFC    0x0004    0x0008
0x0000    0x0004    0x0004
0x0004    0x0004    0x0000    <- trigger interrupt
0x0008    0x0004    0xfffc

Trigger timer interrupt when `delta` is zero or negative (highest bit set).
---
 aclint.c | 24 ++++++++++--------------
 device.h |  3 +--
 main.c   |  9 ++++-----
 riscv.c  |  8 ++++----
 riscv.h  |  3 +--
 5 files changed, 20 insertions(+), 27 deletions(-)

diff --git a/aclint.c b/aclint.c
index aafe8ce..d84176e 100644
--- a/aclint.c
+++ b/aclint.c
@@ -4,13 +4,11 @@
 
 void aclint_timer_interrupts(vm_t *vm, aclint_state_t *aclint)
 {
-    if ((vm->mtimeh > aclint->mtimecmp_hi) ||
-        ((vm->mtimeh == aclint->mtimecmp_hi) &&
-         (vm->mtime >= aclint->mtimecmp_lo)))
+    uint64_t time_delta = aclint->mtimecmp - vm->mtime;
+    if ((time_delta & 0x8000000000000000) || time_delta == 0)
         vm->sip |= RV_INT_STI_BIT;
     else
         vm->sip &= ~RV_INT_STI_BIT;
-    return;
 }
 
 void aclint_update_interrupts(vm_t *vm, aclint_state_t *aclint)
@@ -19,8 +17,6 @@ void aclint_update_interrupts(vm_t *vm, aclint_state_t *aclint)
         vm->sip |= RV_INT_SSI_BIT;
     else
         vm->sip &= ~RV_INT_SSI_BIT;
-
-    return;
 }
 
 static bool aclint_reg_read(aclint_state_t *aclint,
@@ -35,19 +31,19 @@ static bool aclint_reg_read(aclint_state_t *aclint,
         return true;
     case _(MTIMECMP_LO):
         /* mtimecmp */
-        *value = aclint->mtimecmp_lo;
+        *value = aclint->mtimecmp & 0xFFFFFFFF;
         return true;
     case _(MTIMECMP_HI):
         /* mtimecmph */
-        *value = aclint->mtimecmp_hi;
+        *value = (aclint->mtimecmp >> 32) & 0xFFFFFFFF;
         return true;
     case _(MTIME_LO):
         /* mtime */
-        *value = aclint->vm->mtime;
+        *value = aclint->vm->mtime & 0xFFFFFFFF;
         return true;
     case _(MTIME_HI):
         /* mtimeh */
-        *value = aclint->vm->mtimeh;
+        *value = (aclint->vm->mtime >> 32) & 0xFFFFFFFF;
         return true;
     default:
         return false;
@@ -67,19 +63,19 @@ static bool aclint_reg_write(aclint_state_t *aclint,
         return true;
     case _(MTIMECMP_LO):
         /* mtimecmp */
-        aclint->mtimecmp_lo = value;
+        aclint->mtimecmp |= value;
         return true;
     case _(MTIMECMP_HI):
         /* mtimecmph */
-        aclint->mtimecmp_hi = value;
+        aclint->mtimecmp |= ((uint64_t) value) << 32;
         return true;
     case _(MTIME_LO):
         /* mtime */
-        aclint->vm->mtime = value;
+        aclint->vm->mtime |= value;
         return true;
     case _(MTIME_HI):
         /* mtimeh */
-        aclint->vm->mtimeh = value;
+        aclint->vm->mtime |= ((uint64_t) value) << 32;
         return true;
     default:
         return false;
diff --git a/device.h b/device.h
index 470bf83..8cc4915 100644
--- a/device.h
+++ b/device.h
@@ -37,8 +37,7 @@ enum {
 
 typedef struct {
     vm_t *vm;
-    uint32_t mtimecmp_lo;
-    uint32_t mtimecmp_hi;
+    uint64_t mtimecmp;
     uint32_t setssip;
 } aclint_state_t;
 
diff --git a/main.c b/main.c
index 326d0cb..be51604 100644
--- a/main.c
+++ b/main.c
@@ -170,8 +170,8 @@ static inline sbi_ret_t handle_sbi_ecall_TIMER(vm_t *vm, int32_t fid)
     emu_state_t *data = PRIV(vm);
     switch (fid) {
     case SBI_TIMER__SET_TIMER:
-        data->aclint.mtimecmp_lo = vm->x_regs[RV_R_A0];
-        data->aclint.mtimecmp_hi = vm->x_regs[RV_R_A1];
+        data->aclint.mtimecmp = (((uint64_t) vm->x_regs[RV_R_A1])) << 32 |
+                                (uint64_t) vm->x_regs[RV_R_A0];
         return (sbi_ret_t){SBI_SUCCESS, 0};
     default:
         return (sbi_ret_t){SBI_ERR_NOT_SUPPORTED, 0};
@@ -453,12 +453,11 @@ static int semu_start(int argc, char **argv)
 #endif
         }
 
-        if (!++vm.mtime)
-            vm.mtimeh++;
-
         aclint_timer_interrupts(&vm, &emu.aclint);
         aclint_update_interrupts(&vm, &emu.aclint);
 
+        vm.mtime++;
+
         vm_step(&vm);
         if (likely(!vm.error))
             continue;
diff --git a/riscv.c b/riscv.c
index 4dae853..e28fd10 100644
--- a/riscv.c
+++ b/riscv.c
@@ -488,10 +488,10 @@ static void csr_read(vm_t *vm, uint16_t addr, uint32_t *value)
         *value = vm->stval;
         break;
     case RV_CSR_TIME:
-        *value = vm->mtime;
+        *value = vm->mtime & 0xFFFFFFFF;
         break;
     case RV_CSR_TIMEH:
-        *value = vm->mtimeh;
+        *value = (vm->mtime >> 32) & 0xFFFFFFFF;
         break;
     default:
         vm_set_exception(vm, RV_EXC_ILLEGAL_INSN, 0);
@@ -546,10 +546,10 @@ static void csr_write(vm_t *vm, uint16_t addr, uint32_t value)
         vm->stval = value;
         break;
     case RV_CSR_TIME:
-        vm->mtime = value;
+        vm->mtime |= value;
         break;
     case RV_CSR_TIMEH:
-        vm->mtimeh = value;
+        vm->mtime |= ((uint64_t) value) << 32;
         break;
     default:
         vm_set_exception(vm, RV_EXC_ILLEGAL_INSN, 0);
diff --git a/riscv.h b/riscv.h
index 3e9d2b6..4fbfdd7 100644
--- a/riscv.h
+++ b/riscv.h
@@ -104,8 +104,7 @@ struct __vm_internal {
     uint32_t *page_table;
 
     /* Timer */
-    uint32_t mtime;
-    uint32_t mtimeh;
+    uint64_t mtime;
 
     void *priv; /**< environment supplied */