[PATCH v4 06/16] target/riscv: zicfilp `lpad` impl and branch tracking

[Deepak Gupta <debug@rivosinc.com>]

Implements setting lp expected when `jalr` is encountered and implements
`lpad` instruction of zicfilp. `lpad` instruction is taken out of
auipc x0, <imm_20>. This is an existing HINTNOP space. If `lpad` is
target of an indirect branch, cpu checks for 20 bit value in x7 upper
with 20 bit value embedded in `lpad`. If they don't match, cpu raises a
sw check exception with tval = 2.

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>
---
 target/riscv/cpu_user.h                 |  1 +
 target/riscv/insn32.decode              |  5 ++-
 target/riscv/insn_trans/trans_rvi.c.inc | 53 +++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu_user.h b/target/riscv/cpu_user.h
index 02afad608b..e6927ff847 100644
--- a/target/riscv/cpu_user.h
+++ b/target/riscv/cpu_user.h
@@ -15,5 +15,6 @@
 #define xA6 16
 #define xA7 17  /* syscall number for RVI ABI */
 #define xT0 5   /* syscall number for RVE ABI */
+#define xT2 7
 
 #endif
diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
index c45b8fa1d8..494b6cdcc6 100644
--- a/target/riscv/insn32.decode
+++ b/target/riscv/insn32.decode
@@ -123,7 +123,10 @@ sfence_vm   0001000    00100 ..... 000 00000 1110011 @sfence_vm
 
 # *** RV32I Base Instruction Set ***
 lui      ....................       ..... 0110111 @u
-auipc    ....................       ..... 0010111 @u
+{
+  lpad     label:20 00000 0010111
+  auipc    ....................       ..... 0010111 @u
+}
 jal      ....................       ..... 1101111 @j
 jalr     ............     ..... 000 ..... 1100111 @i
 beq      ....... .....    ..... 000 ..... 1100011 @b
diff --git a/target/riscv/insn_trans/trans_rvi.c.inc b/target/riscv/insn_trans/trans_rvi.c.inc
index 98e3806d5e..936b430282 100644
--- a/target/riscv/insn_trans/trans_rvi.c.inc
+++ b/target/riscv/insn_trans/trans_rvi.c.inc
@@ -36,6 +36,47 @@ static bool trans_lui(DisasContext *ctx, arg_lui *a)
     return true;
 }
 
+static bool trans_lpad(DisasContext *ctx, arg_lpad *a)
+{
+    bool lp_expected = ctx->fcfi_lp_expected;
+    /*
+     * If zicfilp not present, the encoding is a nop.
+     * If forward cfi is not enabled, implementation is a nop.
+     */
+    if (!ctx->fcfi_enabled) {
+        return true;
+    }
+
+    if (ctx->base.pc_next == ctx->base.pc_first) {
+        ctx->fcfi_lp_expected = false;
+        /* If landing pad was expected, PC must be 4 byte aligned */
+        if (lp_expected && ((ctx->base.pc_next) & 0x3)) {
+            /*
+             * misaligned, according to spec we should raise sw check exception
+             */
+            gen_helper_raise_sw_check_excep(tcg_env,
+                tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL));
+            return true;
+        }
+    }
+
+    /* if lp was expected and lpad_label non-zero, do label check */
+    if (lp_expected && (a->label != 0)) {
+        TCGLabel *skip = gen_new_label();
+        TCGv tmp = tcg_temp_new();
+        tcg_gen_extract_tl(tmp, get_gpr(ctx, xT2, EXT_NONE), 12, 20);
+        tcg_gen_brcondi_tl(TCG_COND_EQ, tmp, a->label, skip);
+        gen_helper_raise_sw_check_excep(tcg_env,
+            tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL));
+        gen_set_label(skip);
+    }
+
+    tcg_gen_st_tl(tcg_constant_tl(0), tcg_env,
+                  offsetof(CPURISCVState, elp));
+
+    return true;
+}
+
 static bool trans_auipc(DisasContext *ctx, arg_auipc *a)
 {
     TCGv target_pc = dest_gpr(ctx, a->rd);
@@ -75,6 +116,18 @@ static bool trans_jalr(DisasContext *ctx, arg_jalr *a)
     gen_set_gpr(ctx, a->rd, succ_pc);
 
     tcg_gen_mov_tl(cpu_pc, target_pc);
+    if (ctx->fcfi_enabled) {
+        /*
+         * return from functions (i.e. rs1 == xRA || rs1 == xT0) are not
+         * tracked. zicfilp introduces sw guarded branch as well. sw guarded
+         * branch are not tracked. rs1 == xT2 is a sw guarded branch.
+         */
+        if (a->rs1 != xRA && a->rs1 != xT0 && a->rs1 != xT2) {
+            tcg_gen_st_tl(tcg_constant_tl(1),
+                          tcg_env, offsetof(CPURISCVState, elp));
+        }
+    }
+
     lookup_and_goto_ptr(ctx);
 
     if (misaligned) {
-- 
2.44.0