Preliminary support for MMU emulation

To boot a 32-bit RISC-V Linux with MMU, we must first support MMU
emulation. The virtual memory scheme to be supported is SV32. The major
changes in this commit include implementing the MMU-related riscv_io_t
interface and binding it during RISC-V instance initialization. To
reuse the riscv_io_t interface, we modified its prototype.

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.

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 aforementioned page faults.

Related: #310

Author: ChinYikMing

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

@@ -41,18 +41,21 @@ 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 */
+/* RISC-V trap 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 */      \
+#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))(                                                            \
+        _(pagefault_insn, 12)              /* Instruction page fault */               \
+        _(pagefault_load, 13)              /* Load page fault */                      \
+        _(pagefault_store, 15),            /* Store page fault */                     \
+        _(ecall_M, 11)                     /* Environment call from M-mode */         \
     )
 /* clang-format on */
 
@@ -290,7 +293,16 @@ static uint32_t csr_csrrw(riscv_t *rv, uint32_t csr, uint32_t val)
         out &= FFLAG_MASK;
 #endif
 
-    *c = val;
+    if (c == &rv->csr_satp) {
+        const uint8_t mode_sv32 = val >> 31;
+        if (mode_sv32)
+            *c = val & MASK(22); /* store ppn */
+        else                     /* bare mode */
+            *c = 0; /* virtual mem addr maps to same physical mem addr directly
+                     */
+    } else {
+        *c = val;
+    }
 
     return out;
 }
@@ -532,7 +544,7 @@ 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->io.mem_write_w(rv, addr, rv->X[fuse[i].rs2]);
     }
     PC += ir->imm2 * 4;
     if (unlikely(RVOP_NO_NEXT(ir))) {
@@ -556,7 +568,7 @@ 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->X[fuse[i].rd] = rv->io.mem_read_w(rv, addr);
     }
     PC += ir->imm2 * 4;
     if (unlikely(RVOP_NO_NEXT(ir))) {
@@ -666,7 +678,7 @@ 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)) {
@@ -1122,24 +1134,8 @@ void rv_step(void *arg)
 }
 
 #if RV32_HAS(SYSTEM)
-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;
-    while (rv->is_trapped && !rv_has_halted(rv)) {
-        insn = rv->io.mem_ifetch(rv->PC);
-        assert(insn);
-
-        rv_decode(ir, insn);
-        ir->impl = dispatch_table[ir->opcode];
-        rv->compressed = is_compressed(insn);
-        ir->impl(rv, ir, rv->csr_cycle, rv->PC);
-    }
-}
-#endif
+#include "system.c"
+#endif /* SYSTEM */
 
 void ebreak_handler(riscv_t *rv)
 {

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);

src/riscv.c

@@ -165,8 +165,10 @@ void rv_remap_stdstream(riscv_t *rv, fd_stream_pair_t *fsp, uint32_t fsp_size)
 #define MEMIO(op) on_mem_##op
 #define IO_HANDLER_IMPL(type, op, RW)                                     \
     static IIF(RW)(                                                       \
-        /* W */ void MEMIO(op)(riscv_word_t addr, riscv_##type##_t data), \
-        /* R */ riscv_##type##_t MEMIO(op)(riscv_word_t addr))            \
+        /* W */ void MEMIO(op)(UNUSED riscv_t * rv, riscv_word_t addr,    \
+                               riscv_##type##_t data),                    \
+        /* R */ riscv_##type##_t MEMIO(op)(UNUSED riscv_t * rv,           \
+                                           riscv_word_t addr))            \
     {                                                                     \
         IIF(RW)                                                           \
         (memory_##op(addr, (uint8_t *) &data), return memory_##op(addr)); \
@@ -260,9 +262,37 @@ riscv_t *rv_create(riscv_user_t rv_attr)
             .on_memset = memset_handler,
         };
         memcpy(&rv->io, &io, sizeof(riscv_io_t));
-    } else {
-        /* TODO: system emulator */
     }
+#if RV32_HAS(SYSTEM)
+    else {
+        /*
+         * TODO: system emulator
+         * e.g., kernel image, dtb, rootfs
+         *
+         * The test suite is compiled into a single ELF file, so load it as
+         * an ELF executable, just like a userspace ELF.
+         *
+         */
+        elf_t *elf = elf_new();
+        assert(elf && elf_open(elf, (attr->data.system)->elf_program));
+
+        const struct Elf32_Sym *end;
+        if ((end = elf_get_symbol(elf, "_end")))
+            attr->break_addr = end->st_value;
+
+        assert(elf_load(elf, attr->mem));
+
+        /* set the entry pc */
+        const struct Elf32_Ehdr *hdr = get_elf_header(elf);
+        assert(rv_set_pc(rv, hdr->e_entry));
+
+        elf_delete(elf);
+
+        /* this variable has external linkage to mmu_io defined in system.c */
+        extern riscv_io_t mmu_io;
+        memcpy(&rv->io, &mmu_io, sizeof(riscv_io_t));
+    }
+#endif /* SYSTEM */
 
     /* default standard stream.
      * rv_remap_stdstream can be called to overwrite them
@@ -356,7 +386,11 @@ void rv_run(riscv_t *rv)
     assert(rv);
 
     vm_attr_t *attr = PRIV(rv);
+#if RV32_HAS(SYSTEM)
+    assert(attr && attr->data.system && attr->data.system->elf_program);
+#else
     assert(attr && attr->data.user && attr->data.user->elf_program);
+#endif
 
     if (attr->run_flag & RV_RUN_TRACE)
         rv_run_and_trace(rv);
@@ -516,6 +550,9 @@ void rv_reset(riscv_t *rv, riscv_word_t pc)
     /* reset sp pointing to argc */
     rv->X[rv_reg_sp] = stack_top;
 
+    /* reset privilege mode */
+    rv->priv_mode = RV_PRIV_M_MODE;
+
     /* reset the csrs */
     rv->csr_mtvec = 0;
     rv->csr_cycle = 0;
@@ -541,7 +578,6 @@ void rv_reset(riscv_t *rv, riscv_word_t pc)
     rv->csr_misa |= MISA_M;
 #endif
 
-
     rv->halt = false;
 }