target/riscv: Add a property to set vill bit on reserved usage of vsetvli instruction

Usage of vsetvli instruction is reserved if VLMAX is changed when vsetvli rs1
and rd arguments are x0.

In this case, if the new property is true, only the vill bit will be set.

See https://github.com/riscv/riscv-isa-manual/blob/main/src/v-st-ext.adoc#avl-encoding
According to the spec, the above use cases are reserved, and
"Implementations may set vill in either case."

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2422
Signed-off-by: Vasilis Liaskovitis <vliaskovitis@suse.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250618213542.22873-1-vliaskovitis@suse.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -2632,6 +2632,7 @@ static const Property riscv_cpu_properties[] = {
     DEFINE_PROP_BOOL("rvv_ta_all_1s", RISCVCPU, cfg.rvv_ta_all_1s, false),
     DEFINE_PROP_BOOL("rvv_ma_all_1s", RISCVCPU, cfg.rvv_ma_all_1s, false),
     DEFINE_PROP_BOOL("rvv_vl_half_avl", RISCVCPU, cfg.rvv_vl_half_avl, false),
+    DEFINE_PROP_BOOL("rvv_vsetvl_x0_vill", RISCVCPU, cfg.rvv_vsetvl_x0_vill, false),
 
     /*
      * write_misa() is marked as experimental for now so mark

target/riscv/cpu_cfg_fields.h.inc

@@ -114,6 +114,7 @@ BOOL_FIELD(ext_supm)
 BOOL_FIELD(rvv_ta_all_1s)
 BOOL_FIELD(rvv_ma_all_1s)
 BOOL_FIELD(rvv_vl_half_avl)
+BOOL_FIELD(rvv_vsetvl_x0_vill)
 /* Named features  */
 BOOL_FIELD(ext_svade)
 BOOL_FIELD(ext_zic64b)

target/riscv/helper.h

@@ -159,7 +159,7 @@ DEF_HELPER_FLAGS_3(hyp_hsv_d, TCG_CALL_NO_WG, void, env, tl, tl)
 #endif
 
 /* Vector functions */
-DEF_HELPER_3(vsetvl, tl, env, tl, tl)
+DEF_HELPER_4(vsetvl, tl, env, tl, tl, tl)
 DEF_HELPER_5(vle8_v, void, ptr, ptr, tl, env, i32)
 DEF_HELPER_5(vle16_v, void, ptr, ptr, tl, env, i32)
 DEF_HELPER_5(vle32_v, void, ptr, ptr, tl, env, i32)

target/riscv/insn_trans/trans_rvv.c.inc

@@ -202,7 +202,7 @@ static bool do_vsetvl(DisasContext *s, int rd, int rs1, TCGv s2)
         s1 = get_gpr(s, rs1, EXT_ZERO);
     }
 
-    gen_helper_vsetvl(dst, tcg_env, s1, s2);
+    gen_helper_vsetvl(dst, tcg_env, s1, s2, tcg_constant_tl((int) (rd == 0 && rs1 == 0)));
     gen_set_gpr(s, rd, dst);
     finalize_rvv_inst(s);
 
@@ -222,7 +222,7 @@ static bool do_vsetivli(DisasContext *s, int rd, TCGv s1, TCGv s2)
 
     dst = dest_gpr(s, rd);
 
-    gen_helper_vsetvl(dst, tcg_env, s1, s2);
+    gen_helper_vsetvl(dst, tcg_env, s1, s2, tcg_constant_tl(0));
     gen_set_gpr(s, rd, dst);
     finalize_rvv_inst(s);
     gen_update_pc(s, s->cur_insn_len);

target/riscv/vector_helper.c

@@ -35,7 +35,7 @@
 #include <math.h>
 
 target_ulong HELPER(vsetvl)(CPURISCVState *env, target_ulong s1,
-                            target_ulong s2)
+                            target_ulong s2, target_ulong x0)
 {
     int vlmax, vl;
     RISCVCPU *cpu = env_archcpu(env);
@@ -83,6 +83,16 @@ target_ulong HELPER(vsetvl)(CPURISCVState *env, target_ulong s1,
     } else {
         vl = vlmax;
     }
+
+    if (cpu->cfg.rvv_vsetvl_x0_vill && x0 && (env->vl != vl)) {
+        /* only set vill bit. */
+        env->vill = 1;
+        env->vtype = 0;
+        env->vl = 0;
+        env->vstart = 0;
+        return 0;
+    }
+
     env->vl = vl;
     env->vtype = s2;
     env->vstart = 0;