target/riscv: Introduce elp state and enabling controls for zicfilp

zicfilp introduces a new state elp ("expected landing pad") in cpu.
During normal execution, elp is idle (NO_LP_EXPECTED) i.e not expecting
landing pad. On an indirect call, elp moves LP_EXPECTED. When elp is
LP_EXPECTED, only a subsquent landing pad instruction can set state back
to NO_LP_EXPECTED. On reset, elp is set to NO_LP_EXPECTED.

zicfilp is enabled via bit2 in *envcfg CSRs. Enabling control for M-mode
is in mseccfg CSR at bit position 10.

On trap, elp state is saved away in *status.
Adds elp to the migration state as well.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Co-developed-by: Jim Shu <jim.shu@sifive.com>
Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241008225010.1861630-4-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -1011,6 +1011,9 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
     env->menvcfg = 0;
 #endif
 
+    /* on reset elp is clear */
+    env->elp = false;
+
     env->xl = riscv_cpu_mxl(env);
     riscv_cpu_update_mask(env);
     cs->exception_index = RISCV_EXCP_NONE;

target/riscv/cpu.h

@@ -230,6 +230,8 @@ struct CPUArchState {
 
     target_ulong jvt;
 
+    /* elp state for zicfilp extension */
+    bool      elp;
 #ifdef CONFIG_USER_ONLY
     uint32_t elf_flags;
 #endif

target/riscv/cpu_bits.h

@@ -552,6 +552,8 @@
 #define MSTATUS_TVM         0x00100000 /* since: priv-1.10 */
 #define MSTATUS_TW          0x00200000 /* since: priv-1.10 */
 #define MSTATUS_TSR         0x00400000 /* since: priv-1.10 */
+#define MSTATUS_SPELP       0x00800000 /* zicfilp */
+#define MSTATUS_MPELP       0x020000000000 /* zicfilp */
 #define MSTATUS_GVA         0x4000000000ULL
 #define MSTATUS_MPV         0x8000000000ULL
 
@@ -582,6 +584,7 @@ typedef enum {
 #define SSTATUS_XS          0x00018000
 #define SSTATUS_SUM         0x00040000 /* since: priv-1.10 */
 #define SSTATUS_MXR         0x00080000
+#define SSTATUS_SPELP       MSTATUS_SPELP   /* zicfilp */
 
 #define SSTATUS64_UXL       0x0000000300000000ULL
 
@@ -754,6 +757,7 @@ typedef enum RISCVException {
 
 /* Execution environment configuration bits */
 #define MENVCFG_FIOM                       BIT(0)
+#define MENVCFG_LPE                        BIT(2) /* zicfilp */
 #define MENVCFG_CBIE                       (3UL << 4)
 #define MENVCFG_CBCFE                      BIT(6)
 #define MENVCFG_CBZE                       BIT(7)
@@ -767,11 +771,13 @@ typedef enum RISCVException {
 #define MENVCFGH_STCE                      BIT(31)
 
 #define SENVCFG_FIOM                       MENVCFG_FIOM
+#define SENVCFG_LPE                        MENVCFG_LPE
 #define SENVCFG_CBIE                       MENVCFG_CBIE
 #define SENVCFG_CBCFE                      MENVCFG_CBCFE
 #define SENVCFG_CBZE                       MENVCFG_CBZE
 
 #define HENVCFG_FIOM                       MENVCFG_FIOM
+#define HENVCFG_LPE                        MENVCFG_LPE
 #define HENVCFG_CBIE                       MENVCFG_CBIE
 #define HENVCFG_CBCFE                      MENVCFG_CBCFE
 #define HENVCFG_CBZE                       MENVCFG_CBZE

target/riscv/csr.c

@@ -1598,6 +1598,11 @@ static RISCVException write_mstatus(CPURISCVState *env, int csrno,
         }
     }
 
+    /* If cfi lp extension is available, then apply cfi lp mask */
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        mask |= (MSTATUS_MPELP | MSTATUS_SPELP);
+    }
+
     mstatus = (mstatus & ~mask) | (val & mask);
 
     env->mstatus = mstatus;
@@ -2344,6 +2349,10 @@ static RISCVException write_menvcfg(CPURISCVState *env, int csrno,
         mask |= (cfg->ext_svpbmt ? MENVCFG_PBMTE : 0) |
                 (cfg->ext_sstc ? MENVCFG_STCE : 0) |
                 (cfg->ext_svadu ? MENVCFG_ADUE : 0);
+
+        if (env_archcpu(env)->cfg.ext_zicfilp) {
+            mask |= MENVCFG_LPE;
+        }
     }
     env->menvcfg = (env->menvcfg & ~mask) | (val & mask);
 
@@ -2396,6 +2405,10 @@ static RISCVException write_senvcfg(CPURISCVState *env, int csrno,
         return ret;
     }
 
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        mask |= SENVCFG_LPE;
+    }
+
     env->senvcfg = (env->senvcfg & ~mask) | (val & mask);
     return RISCV_EXCP_NONE;
 }
@@ -2433,6 +2446,10 @@ static RISCVException write_henvcfg(CPURISCVState *env, int csrno,
 
     if (riscv_cpu_mxl(env) == MXL_RV64) {
         mask |= env->menvcfg & (HENVCFG_PBMTE | HENVCFG_STCE | HENVCFG_ADUE);
+
+        if (env_archcpu(env)->cfg.ext_zicfilp) {
+            mask |= HENVCFG_LPE;
+        }
     }
 
     env->henvcfg = (env->henvcfg & ~mask) | (val & mask);
@@ -2897,6 +2914,10 @@ static RISCVException read_sstatus_i128(CPURISCVState *env, int csrno,
         mask |= SSTATUS64_UXL;
     }
 
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        mask |= SSTATUS_SPELP;
+    }
+
     *val = int128_make128(sstatus, add_status_sd(MXL_RV128, sstatus));
     return RISCV_EXCP_NONE;
 }
@@ -2908,6 +2929,11 @@ static RISCVException read_sstatus(CPURISCVState *env, int csrno,
     if (env->xl != MXL_RV32 || env->debugger) {
         mask |= SSTATUS64_UXL;
     }
+
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        mask |= SSTATUS_SPELP;
+    }
+
     /* TODO: Use SXL not MXL. */
     *val = add_status_sd(riscv_cpu_mxl(env), env->mstatus & mask);
     return RISCV_EXCP_NONE;
@@ -2923,6 +2949,11 @@ static RISCVException write_sstatus(CPURISCVState *env, int csrno,
             mask |= SSTATUS64_UXL;
         }
     }
+
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        mask |= SSTATUS_SPELP;
+    }
+
     target_ulong newval = (env->mstatus & ~mask) | (val & mask);
     return write_mstatus(env, CSR_MSTATUS, newval);
 }

target/riscv/machine.c

@@ -350,6 +350,24 @@ static const VMStateDescription vmstate_jvt = {
     }
 };
 
+static bool elp_needed(void *opaque)
+{
+    RISCVCPU *cpu = opaque;
+
+    return cpu->cfg.ext_zicfilp;
+}
+
+static const VMStateDescription vmstate_elp = {
+    .name = "cpu/elp",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = elp_needed,
+    .fields = (const VMStateField[]) {
+        VMSTATE_BOOL(env.elp, RISCVCPU),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 const VMStateDescription vmstate_riscv_cpu = {
     .name = "cpu",
     .version_id = 10,
@@ -422,6 +440,7 @@ const VMStateDescription vmstate_riscv_cpu = {
         &vmstate_debug,
         &vmstate_smstateen,
         &vmstate_jvt,
+        &vmstate_elp,
         NULL
     }
 };

target/riscv/pmp.c

@@ -598,6 +598,11 @@ void mseccfg_csr_write(CPURISCVState *env, target_ulong val)
         val &= ~(MSECCFG_MMWP | MSECCFG_MML | MSECCFG_RLB);
     }
 
+    /* M-mode forward cfi to be enabled if cfi extension is implemented */
+    if (env_archcpu(env)->cfg.ext_zicfilp) {
+        val |= (val & MSECCFG_MLPE);
+    }
+
     env->mseccfg = val;
 }
 

target/riscv/pmp.h

@@ -44,7 +44,8 @@ typedef enum {
     MSECCFG_MMWP  = 1 << 1,
     MSECCFG_RLB   = 1 << 2,
     MSECCFG_USEED = 1 << 8,
-    MSECCFG_SSEED = 1 << 9
+    MSECCFG_SSEED = 1 << 9,
+    MSECCFG_MLPE =  1 << 10,
 } mseccfg_field_t;
 
 typedef struct {