RISC-V: Add support for the Zifencei extension

fence.i has been split out of the base ISA as part of the ratification
process.  This patch adds a Zifencei argument, which disables the
fence.i instruction.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -441,6 +441,7 @@ static Property riscv_cpu_properties[] = {
     DEFINE_PROP_BOOL("s", RISCVCPU, cfg.ext_s, true),
     DEFINE_PROP_BOOL("u", RISCVCPU, cfg.ext_u, true),
     DEFINE_PROP_BOOL("Counters", RISCVCPU, cfg.ext_counters, true),
+    DEFINE_PROP_BOOL("Zifencei", RISCVCPU, cfg.ext_ifencei, true),
     DEFINE_PROP_STRING("priv_spec", RISCVCPU, cfg.priv_spec),
     DEFINE_PROP_BOOL("mmu", RISCVCPU, cfg.mmu, true),
     DEFINE_PROP_BOOL("pmp", RISCVCPU, cfg.pmp, true),

target/riscv/cpu.h

@@ -223,6 +223,7 @@ typedef struct RISCVCPU {
         bool ext_s;
         bool ext_u;
         bool ext_counters;
+        bool ext_ifencei;
 
         char *priv_spec;
         char *user_spec;

target/riscv/insn_trans/trans_rvi.inc.c

@@ -484,6 +484,10 @@ static bool trans_fence(DisasContext *ctx, arg_fence *a)
 
 static bool trans_fence_i(DisasContext *ctx, arg_fence_i *a)
 {
+    if (!ctx->ext_ifencei) {
+        return false;
+    }
+
     /*
      * FENCE_I is a no-op in QEMU,
      * however we need to end the translation block

target/riscv/translate.c

@@ -54,6 +54,7 @@ typedef struct DisasContext {
        to any system register, which includes CSR_FRM, so we do not have
        to reset this known value.  */
     int frm;
+    bool ext_ifencei;
 } DisasContext;
 
 #ifdef TARGET_RISCV64
@@ -752,13 +753,15 @@ static void riscv_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
 {
     DisasContext *ctx = container_of(dcbase, DisasContext, base);
     CPURISCVState *env = cs->env_ptr;
+    RISCVCPU *cpu = RISCV_CPU(cs);
 
     ctx->pc_succ_insn = ctx->base.pc_first;
     ctx->mem_idx = ctx->base.tb->flags & TB_FLAGS_MMU_MASK;
     ctx->mstatus_fs = ctx->base.tb->flags & TB_FLAGS_MSTATUS_FS;
     ctx->priv_ver = env->priv_ver;
     ctx->misa = env->misa;
     ctx->frm = -1;  /* unknown rounding mode */
+    ctx->ext_ifencei = cpu->cfg.ext_ifencei;
 }
 
 static void riscv_tr_tb_start(DisasContextBase *db, CPUState *cpu)