RISC-V: Add support for the Zicsr extension

The various CSR instructions have been split out of the base ISA as part
of the ratification process.  This patch adds a Zicsr argument, which
disables all the CSR instructions.

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

target/riscv/cpu.c

@@ -442,6 +442,7 @@ static Property riscv_cpu_properties[] = {
     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_BOOL("Zicsr", RISCVCPU, cfg.ext_icsr, 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

@@ -224,6 +224,7 @@ typedef struct RISCVCPU {
         bool ext_u;
         bool ext_counters;
         bool ext_ifencei;
+        bool ext_icsr;
 
         char *priv_spec;
         char *user_spec;

target/riscv/csr.c

@@ -793,6 +793,7 @@ int riscv_csrrw(CPURISCVState *env, int csrno, target_ulong *ret_value,
 {
     int ret;
     target_ulong old_value;
+    RISCVCPU *cpu = env_archcpu(env);
 
     /* check privileges and return -1 if check fails */
 #if !defined(CONFIG_USER_ONLY)
@@ -803,6 +804,11 @@ int riscv_csrrw(CPURISCVState *env, int csrno, target_ulong *ret_value,
     }
 #endif
 
+    /* ensure the CSR extension is enabled. */
+    if (!cpu->cfg.ext_icsr) {
+        return -1;
+    }
+
     /* check predicate */
     if (!csr_ops[csrno].predicate || csr_ops[csrno].predicate(env, csrno) < 0) {
         return -1;