Preliminary support for MMU emulation

To boot a 32-bit RISC-V Linux with MMU, MMU emulation support is
crucial, using the SV32 virtual memory scheme. This commit’s main
changes include implementing the MMU-related riscv_io_t interface and
binding it during RISC-V instance initialization. To enable reuse of
riscv_io_t, its prototype is modified to accept the RISC-V core
instance as the first parameter, allowing MMU-enabled I/O to access
the SATP CSR. A trap_handler callback is also added to the riscv_io_t
interface. This keeps the dispatch_table and _trap_handler static
within emulate.c, aligning the schema with other handlers, like
ebreak_handler and ecall_handler. With m/scause and m/stval set
in SET_CAUSE_AND_TVAL_THEN_TRAP, _trap_handler can immediately
dispatch without rechecking scause, avoiding the need for additional
call of specific type of trap handler, thus no more need of
TRAP_HANDLER_IMPL.

For each memory access, the page table is walked to get the
corresponding PTE. Depending on the PTE retrieval, several page faults
may need handling. Thus, three exception handlers have been introduced:
insn_pgfault, load_pgfault, and store_pgfault, used in MMU_CHECK_FAULT.
This commit does not fully handle access faults since they are related
to PMA and PMP, which may not be necessary for booting 32-bit RISC-V
Linux (possibly supported in the future).

Since Linux has not been booted yet, a test suite is needed to test the
MMU emulation. This commit includes a test suite that implements a
simple kernel space supervisor and a user space application. The
supervisor prepares the page table and then passes control to the user
space application to test the three aforementioned page faults.

Related: #310

Author: ChinYikMing

Makefile

@@ -45,6 +45,10 @@ CFLAGS += $(CFLAGS_NO_CET)
 
 OBJS_EXT :=
 
+ifeq ($(call has, SYSTEM), 1)
+OBJS_EXT += system.o
+endif
+
 # Integer Multiplication and Division instructions
 ENABLE_EXT_M ?= 1
 $(call set-feature, EXT_M)

src/decode.c

@@ -907,9 +907,8 @@ static inline bool op_system(rv_insn_t *ir, const uint32_t insn)
     default: /* illegal instruction */
         return false;
     }
-    if (!csr_is_writable(ir->imm) && ir->rs1 != rv_reg_zero)
-        return false;
-    return true;
+
+    return csr_is_writable(ir->imm) || (ir->rs1 == rv_reg_zero);
 }
 
 /* MISC-MEM: I-type

src/emulate.c

@@ -14,6 +14,10 @@
 #include <emscripten.h>
 #endif
 
+#if RV32_HAS(SYSTEM)
+#include "system.h"
+#endif /* RV32_HAS(SYSTEM) */
+
 #if RV32_HAS(EXT_F)
 #include <math.h>
 #include "softfloat.h"
@@ -41,130 +45,15 @@ extern struct target_ops gdbstub_ops;
 #define IF_rs2(i, r) (i->rs2 == rv_reg_##r)
 #define IF_imm(i, v) (i->imm == v)
 
-/* RISC-V exception code list */
-/* clang-format off */
-#define RV_TRAP_LIST                                               \
-    IIF(RV32_HAS(EXT_C))(,                                         \
-        _(insn_misaligned, 0) /* Instruction address misaligned */ \
-    )                                                              \
-    _(illegal_insn, 2)     /* Illegal instruction */               \
-    _(breakpoint, 3)       /* Breakpoint */                        \
-    _(load_misaligned, 4)  /* Load address misaligned */           \
-    _(store_misaligned, 6) /* Store/AMO address misaligned */      \
-    IIF(RV32_HAS(SYSTEM))(,                                        \
-        _(ecall_M, 11)     /* Environment call from M-mode */      \
-    )
-/* clang-format on */
-
-enum {
-#define _(type, code) rv_trap_code_##type = code,
-    RV_TRAP_LIST
-#undef _
-};
-
 static void rv_trap_default_handler(riscv_t *rv)
 {
     rv->csr_mepc += rv->compressed ? 2 : 4;
     rv->PC = rv->csr_mepc; /* mret */
 }
 
-/*
- * Trap might occurs during block emulation. For instance, page fault.
- * In order to handle trap, we have to escape from block and execute
- * registered trap handler. This trap_handler function helps to execute
- * the registered trap handler, PC by PC. Once the trap is handled,
- * resume the previous execution flow where cause the trap.
- *
- * Since the system emulation has not yet included in rv32emu, the page
- * fault is not practical in current test suite. Instead, we try to
- * emulate the misaligned handling in the test suite.
- */
 #if RV32_HAS(SYSTEM)
-static void trap_handler(riscv_t *rv);
-#endif
-
-/* When a trap occurs in M-mode/S-mode, m/stval is either initialized to zero or
- * populated with exception-specific details to assist software in managing
- * the trap. Otherwise, the implementation never modifies m/stval, although
- * software can explicitly write to it. The hardware platform will define
- * which exceptions are required to informatively set mtval and which may
- * consistently set it to zero.
- *
- * When a hardware breakpoint is triggered or an exception like address
- * misalignment, access fault, or page fault occurs during an instruction
- * fetch, load, or store operation, m/stval is updated with the virtual address
- * that caused the fault. In the case of an illegal instruction trap, m/stval
- * might be updated with the first XLEN or ILEN bits of the offending
- * instruction. For all other traps, m/stval is simply set to zero. However,
- * it is worth noting that a future standard could redefine how m/stval is
- * handled for different types of traps.
- *
- * For simplicity and clarity, abstracting stval and mtval into a single
- * identifier called tval, as both are handled by TRAP_HANDLER_IMPL.
- */
-#define TRAP_HANDLER_IMPL(type, code)                                         \
-    static void rv_trap_##type(riscv_t *rv, uint32_t tval)                    \
-    {                                                                         \
-        /* m/stvec (Machine/Supervisor Trap-Vector Base Address Register)     \
-         * m/stvec[MXLEN-1:2]: vector base address                            \
-         * m/stvec[1:0] : vector mode                                         \
-         * m/sepc  (Machine/Supervisor Exception Program Counter)             \
-         * m/stval (Machine/Supervisor Trap Value Register)                   \
-         * m/scause (Machine/Supervisor Cause Register): store exception code \
-         * m/sstatus (Machine/Supervisor Status Register): keep track of and  \
-         * controls the hart’s current operating state                      \
-         */                                                                   \
-        uint32_t base;                                                        \
-        uint32_t mode;                                                        \
-        /* user or supervisor */                                              \
-        if (RV_PRIV_IS_U_OR_S_MODE()) {                                       \
-            const uint32_t sstatus_sie =                                      \
-                (rv->csr_sstatus & SSTATUS_SIE) >> SSTATUS_SIE_SHIFT;         \
-            rv->csr_sstatus |= (sstatus_sie << SSTATUS_SPIE_SHIFT);           \
-            rv->csr_sstatus &= ~(SSTATUS_SIE);                                \
-            rv->csr_sstatus |= (rv->priv_mode << SSTATUS_SPP_SHIFT);          \
-            rv->priv_mode = RV_PRIV_S_MODE;                                   \
-            base = rv->csr_stvec & ~0x3;                                      \
-            mode = rv->csr_stvec & 0x3;                                       \
-            rv->csr_sepc = rv->PC;                                            \
-            rv->csr_stval = tval;                                             \
-            rv->csr_scause = code;                                            \
-        } else { /* machine */                                                \
-            const uint32_t mstatus_mie =                                      \
-                (rv->csr_mstatus & MSTATUS_MIE) >> MSTATUS_MIE_SHIFT;         \
-            rv->csr_mstatus |= (mstatus_mie << MSTATUS_MPIE_SHIFT);           \
-            rv->csr_mstatus &= ~(MSTATUS_MIE);                                \
-            rv->csr_mstatus |= (rv->priv_mode << MSTATUS_MPP_SHIFT);          \
-            rv->priv_mode = RV_PRIV_M_MODE;                                   \
-            base = rv->csr_mtvec & ~0x3;                                      \
-            mode = rv->csr_mtvec & 0x3;                                       \
-            rv->csr_mepc = rv->PC;                                            \
-            rv->csr_mtval = tval;                                             \
-            rv->csr_mcause = code;                                            \
-            if (!rv->csr_mtvec) { /* in case CSR is not configured */         \
-                rv_trap_default_handler(rv);                                  \
-                return;                                                       \
-            }                                                                 \
-        }                                                                     \
-        switch (mode) {                                                       \
-        /* DIRECT: All traps set PC to base */                                \
-        case 0:                                                               \
-            rv->PC = base;                                                    \
-            break;                                                            \
-        /* VECTORED: Asynchronous traps set PC to base + 4 * code */          \
-        case 1:                                                               \
-            /* MSB of code is used to indicate whether the trap is interrupt  \
-             * or exception, so it is not considered as the 'real' code */    \
-            rv->PC = base + 4 * (code & MASK(31));                            \
-            break;                                                            \
-        }                                                                     \
-        IIF(RV32_HAS(SYSTEM))(if (rv->is_trapped) trap_handler(rv);, )        \
-    }
-
-/* RISC-V exception handlers */
-#define _(type, code) TRAP_HANDLER_IMPL(type, code)
-RV_TRAP_LIST
-#undef _
+static void __trap_handler(riscv_t *rv);
+#endif /* SYSTEM */
 
 /* wrap load/store and insn misaligned handler
  * @mask_or_pc: mask for load/store and pc for insn misaligned handler.
@@ -180,8 +69,8 @@ RV_TRAP_LIST
         rv->compressed = compress;                                    \
         rv->csr_cycle = cycle;                                        \
         rv->PC = PC;                                                  \
-        IIF(RV32_HAS(SYSTEM))(rv->is_trapped = true, );               \
-        rv_trap_##type##_misaligned(rv, IIF(IO)(addr, mask_or_pc));   \
+        SET_CAUSE_AND_TVAL_THEN_TRAP(rv, type##_MISALIGNED,           \
+                                     IIF(IO)(addr, mask_or_pc));      \
         return false;                                                 \
     }
 
@@ -531,8 +420,8 @@ static bool do_fuse3(riscv_t *rv, rv_insn_t *ir, uint64_t cycle, uint32_t PC)
      */
     for (int i = 0; i < ir->imm2; i++) {
         uint32_t addr = rv->X[fuse[i].rs1] + fuse[i].imm;
-        RV_EXC_MISALIGN_HANDLER(3, store, false, 1);
-        rv->io.mem_write_w(addr, rv->X[fuse[i].rs2]);
+        RV_EXC_MISALIGN_HANDLER(3, STORE, false, 1);
+        rv->io.mem_write_w(rv, addr, rv->X[fuse[i].rs2]);
     }
     PC += ir->imm2 * 4;
     if (unlikely(RVOP_NO_NEXT(ir))) {
@@ -555,8 +444,8 @@ static bool do_fuse4(riscv_t *rv, rv_insn_t *ir, uint64_t cycle, uint32_t PC)
      */
     for (int i = 0; i < ir->imm2; i++) {
         uint32_t addr = rv->X[fuse[i].rs1] + fuse[i].imm;
-        RV_EXC_MISALIGN_HANDLER(3, load, false, 1);
-        rv->X[fuse[i].rd] = rv->io.mem_read_w(addr);
+        RV_EXC_MISALIGN_HANDLER(3, LOAD, false, 1);
+        rv->X[fuse[i].rd] = rv->io.mem_read_w(rv, addr);
     }
     PC += ir->imm2 * 4;
     if (unlikely(RVOP_NO_NEXT(ir))) {
@@ -666,12 +555,12 @@ static void block_translate(riscv_t *rv, block_t *block)
             prev_ir->next = ir;
 
         /* fetch the next instruction */
-        const uint32_t insn = rv->io.mem_ifetch(block->pc_end);
+        const uint32_t insn = rv->io.mem_ifetch(rv, block->pc_end);
 
         /* decode the instruction */
         if (!rv_decode(ir, insn)) {
             rv->compressed = is_compressed(insn);
-            rv_trap_illegal_insn(rv, insn);
+            SET_CAUSE_AND_TVAL_THEN_TRAP(rv, ILLEGAL_INSN, insn);
             break;
         }
         ir->impl = dispatch_table[ir->opcode];
@@ -1122,15 +1011,14 @@ void rv_step(void *arg)
 }
 
 #if RV32_HAS(SYSTEM)
-static void trap_handler(riscv_t *rv)
+static void __trap_handler(riscv_t *rv)
 {
     rv_insn_t *ir = mpool_alloc(rv->block_ir_mp);
     assert(ir);
 
-    /* set to false by sret/mret implementation */
-    uint32_t insn;
+    /* set to false by sret implementation */
     while (rv->is_trapped && !rv_has_halted(rv)) {
-        insn = rv->io.mem_ifetch(rv->PC);
+        uint32_t insn = rv->io.mem_ifetch(rv, rv->PC);
         assert(insn);
 
         rv_decode(ir, insn);
@@ -1139,12 +1027,94 @@ static void trap_handler(riscv_t *rv)
         ir->impl(rv, ir, rv->csr_cycle, rv->PC);
     }
 }
-#endif
+#endif /* SYSTEM */
+
+/* When a trap occurs in M-mode/S-mode, m/stval is either initialized to zero or
+ * populated with exception-specific details to assist software in managing
+ * the trap. Otherwise, the implementation never modifies m/stval, although
+ * software can explicitly write to it. The hardware platform will define
+ * which exceptions are required to informatively set mtval and which may
+ * consistently set it to zero.
+ *
+ * When a hardware breakpoint is triggered or an exception like address
+ * misalignment, access fault, or page fault occurs during an instruction
+ * fetch, load, or store operation, m/stval is updated with the virtual address
+ * that caused the fault. In the case of an illegal instruction trap, m/stval
+ * might be updated with the first XLEN or ILEN bits of the offending
+ * instruction. For all other traps, m/stval is simply set to zero. However,
+ * it is worth noting that a future standard could redefine how m/stval is
+ * handled for different types of traps.
+ *
+ */
+static void _trap_handler(riscv_t *rv)
+{
+    /* m/stvec (Machine/Supervisor Trap-Vector Base Address Register)
+     * m/stvec[MXLEN-1:2]: vector base address
+     * m/stvec[1:0] : vector mode
+     * m/sepc  (Machine/Supervisor Exception Program Counter)
+     * m/stval (Machine/Supervisor Trap Value Register)
+     * m/scause (Machine/Supervisor Cause Register): store exception code
+     * m/sstatus (Machine/Supervisor Status Register): keep track of and
+     * controls the hart’s current operating state
+     *
+     * m/stval and m/scause are set in SET_CAUSE_AND_TVAL_THEN_TRAP
+     */
+    uint32_t base;
+    uint32_t mode;
+    uint32_t cause;
+    /* user or supervisor */
+    if (RV_PRIV_IS_U_OR_S_MODE()) {
+        const uint32_t sstatus_sie =
+            (rv->csr_sstatus & SSTATUS_SIE) >> SSTATUS_SIE_SHIFT;
+        rv->csr_sstatus |= (sstatus_sie << SSTATUS_SPIE_SHIFT);
+        rv->csr_sstatus &= ~(SSTATUS_SIE);
+        rv->csr_sstatus |= (rv->priv_mode << SSTATUS_SPP_SHIFT);
+        rv->priv_mode = RV_PRIV_S_MODE;
+        base = rv->csr_stvec & ~0x3;
+        mode = rv->csr_stvec & 0x3;
+        cause = rv->csr_scause;
+        rv->csr_sepc = rv->PC;
+    } else { /* machine */
+        const uint32_t mstatus_mie =
+            (rv->csr_mstatus & MSTATUS_MIE) >> MSTATUS_MIE_SHIFT;
+        rv->csr_mstatus |= (mstatus_mie << MSTATUS_MPIE_SHIFT);
+        rv->csr_mstatus &= ~(MSTATUS_MIE);
+        rv->csr_mstatus |= (rv->priv_mode << MSTATUS_MPP_SHIFT);
+        rv->priv_mode = RV_PRIV_M_MODE;
+        base = rv->csr_mtvec & ~0x3;
+        mode = rv->csr_mtvec & 0x3;
+        cause = rv->csr_mcause;
+        rv->csr_mepc = rv->PC;
+        if (!rv->csr_mtvec) { /* in case CSR is not configured */
+            rv_trap_default_handler(rv);
+            return;
+        }
+    }
+    switch (mode) {
+    /* DIRECT: All traps set PC to base */
+    case 0:
+        rv->PC = base;
+        break;
+    /* VECTORED: Asynchronous traps set PC to base + 4 * code */
+    case 1:
+        /* MSB of code is used to indicate whether the trap is interrupt
+         * or exception, so it is not considered as the 'real' code */
+        rv->PC = base + 4 * (cause & MASK(31));
+        break;
+    }
+    IIF(RV32_HAS(SYSTEM))(if (rv->is_trapped) __trap_handler(rv);, )
+}
+
+void trap_handler(riscv_t *rv)
+{
+    assert(rv);
+    _trap_handler(rv);
+}
 
 void ebreak_handler(riscv_t *rv)
 {
     assert(rv);
-    rv_trap_breakpoint(rv, rv->PC);
+    SET_CAUSE_AND_TVAL_THEN_TRAP(rv, BREAKPOINT, rv->PC);
 }
 
 void ecall_handler(riscv_t *rv)
@@ -1154,7 +1124,7 @@ void ecall_handler(riscv_t *rv)
     syscall_handler(rv);
     rv->PC += 4;
 #else
-    rv_trap_ecall_M(rv, 0);
+    SET_CAUSE_AND_TVAL_THEN_TRAP(rv, ECALL_M, 0);
     syscall_handler(rv);
 #endif
 }

src/feature.h

@@ -63,5 +63,10 @@
 #define RV32_FEATURE_T2C 0
 #endif
 
+/* System */
+#ifndef RV32_FEATURE_SYSTEM
+#define RV32_FEATURE_SYSTEM 0
+#endif
+
 /* Feature test macro */
 #define RV32_HAS(x) RV32_FEATURE_##x

src/gdbstub.c

@@ -55,7 +55,7 @@ static int rv_read_mem(void *args, size_t addr, size_t len, void *val)
          * an invalid address. We may have to do error handling in the
          * mem_read_* function directly.
          */
-        *((uint8_t *) val + i) = rv->io.mem_read_b(addr + i);
+        *((uint8_t *) val + i) = rv->io.mem_read_b(rv, addr + i);
     }
 
     return err;
@@ -66,7 +66,7 @@ static int rv_write_mem(void *args, size_t addr, size_t len, void *val)
     riscv_t *rv = (riscv_t *) args;
 
     for (size_t i = 0; i < len; i++)
-        rv->io.mem_write_b(addr + i, *((uint8_t *) val + i));
+        rv->io.mem_write_b(rv, addr + i, *((uint8_t *) val + i));
 
     return 0;
 }

src/main.c

@@ -217,12 +217,21 @@ int main(int argc, char **args)
         .log_level = 0,
         .run_flag = run_flag,
         .profile_output_file = prof_out_file,
+#if RV32_HAS(SYSTEM)
+        .data.system = malloc(sizeof(vm_system_t)),
+#else
         .data.user = malloc(sizeof(vm_user_t)),
+#endif
         .cycle_per_step = CYCLE_PER_STEP,
         .allow_misalign = opt_misaligned,
     };
+#if RV32_HAS(SYSTEM)
+    assert(attr.data.system);
+    attr.data.system->elf_program = opt_prog_name;
+#else
     assert(attr.data.user);
     attr.data.user->elf_program = opt_prog_name;
+#endif
 
     /* create the RISC-V runtime */
     rv = rv_create(&attr);