target/riscv: Fix MEPC/SEPC bit masking for IALIGN

According to the RISC-V Privileged Architecture specification, the low
bit of MEPC/SEPC must always be zero. When IALIGN=32, the two low bits
must be zero.

This commit fixes the behavior of MEPC/SEPC CSR reads and writes, and
the implicit reads by MRET/SRET instructions to properly mask the
lowest bit(s) based on whether the C extension is enabled:
- When C extension is enabled (IALIGN=16): mask bit 0
- When C extension is disabled (IALIGN=32): mask bits [1:0]

Previously, when vectored mode bits from STVEC (which sets bit 0 for
vectored mode) were written to MEPC, the bits would not be cleared
correctly, causing incorrect behavior on MRET.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2855
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250703182157.281320-2-charmitro@posteo.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/csr.c

@@ -3129,14 +3129,14 @@ static RISCVException write_mscratch(CPURISCVState *env, int csrno,
 static RISCVException read_mepc(CPURISCVState *env, int csrno,
                                 target_ulong *val)
 {
-    *val = env->mepc;
+    *val = env->mepc & get_xepc_mask(env);
     return RISCV_EXCP_NONE;
 }
 
 static RISCVException write_mepc(CPURISCVState *env, int csrno,
                                  target_ulong val, uintptr_t ra)
 {
-    env->mepc = val;
+    env->mepc = val & get_xepc_mask(env);
     return RISCV_EXCP_NONE;
 }
 
@@ -4169,14 +4169,14 @@ static RISCVException write_sscratch(CPURISCVState *env, int csrno,
 static RISCVException read_sepc(CPURISCVState *env, int csrno,
                                 target_ulong *val)
 {
-    *val = env->sepc;
+    *val = env->sepc & get_xepc_mask(env);
     return RISCV_EXCP_NONE;
 }
 
 static RISCVException write_sepc(CPURISCVState *env, int csrno,
                                  target_ulong val, uintptr_t ra)
 {
-    env->sepc = val;
+    env->sepc = val & get_xepc_mask(env);
     return RISCV_EXCP_NONE;
 }
 

target/riscv/internals.h

@@ -158,6 +158,17 @@ static inline float16 check_nanbox_bf16(CPURISCVState *env, uint64_t f)
     }
 }
 
+static inline target_ulong get_xepc_mask(CPURISCVState *env)
+{
+    /* When IALIGN=32, both low bits must be zero.
+     * When IALIGN=16 (has C extension), only bit 0 must be zero. */
+    if (riscv_has_ext(env, RVC)) {
+        return ~(target_ulong)1;
+    } else {
+        return ~(target_ulong)3;
+    }
+}
+
 #ifndef CONFIG_USER_ONLY
 /* Our implementation of SysemuCPUOps::has_work */
 bool riscv_cpu_has_work(CPUState *cs);

target/riscv/op_helper.c

@@ -280,7 +280,7 @@ target_ulong helper_sret(CPURISCVState *env)
         riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
     }
 
-    target_ulong retpc = env->sepc;
+    target_ulong retpc = env->sepc & get_xepc_mask(env);
     if (!riscv_cpu_allow_16bit_insn(&env_archcpu(env)->cfg,
                                     env->priv_ver,
                                     env->misa_ext) && (retpc & 0x3)) {
@@ -391,7 +391,7 @@ static target_ulong ssdbltrp_mxret(CPURISCVState *env, target_ulong mstatus,
 
 target_ulong helper_mret(CPURISCVState *env)
 {
-    target_ulong retpc = env->mepc;
+    target_ulong retpc = env->mepc & get_xepc_mask(env);
     uint64_t mstatus = env->mstatus;
     target_ulong prev_priv = get_field(mstatus, MSTATUS_MPP);