[PULL 03/14] target/arm: Enforce that M-profile SP low 2 bits are always zero

[Peter Maydell <peter.maydell@linaro.org>]

For M-profile, unlike A-profile, the low 2 bits of SP are defined to be
RES0H, which is to say that they must be hardwired to zero so that
guest attempts to write non-zero values to them are ignored.

Implement this behaviour by masking out the low bits:
 * for writes to r13 by the gdbstub
 * for writes to any of the various flavours of SP via MSR
 * for writes to r13 via store_reg() in generated code

Note that all the direct uses of cpu_R[] in translate.c are in places
where the register is definitely not r13 (usually because that has
been checked for as an UNDEFINED or UNPREDICTABLE case and handled as
UNDEF).

All the other writes to regs[13] in C code are either:
 * A-profile only code
 * writes of values we can guarantee to be aligned, such as
   - writes of previous-SP-value plus or minus a 4-aligned constant
   - writes of the value in an SP limit register (which we already
     enforce to be aligned)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210723162146.5167-2-peter.maydell@linaro.org
---
 target/arm/gdbstub.c   |  4 ++++
 target/arm/m_helper.c  | 14 ++++++++------
 target/arm/translate.c |  3 +++
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/target/arm/gdbstub.c b/target/arm/gdbstub.c
index a8fff2a3d09..826601b3415 100644
--- a/target/arm/gdbstub.c
+++ b/target/arm/gdbstub.c
@@ -84,6 +84,10 @@ int arm_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)
 
     if (n < 16) {
         /* Core integer register.  */
+        if (n == 13 && arm_feature(env, ARM_FEATURE_M)) {
+            /* M profile SP low bits are always 0 */
+            tmp &= ~3;
+        }
         env->regs[n] = tmp;
         return 4;
     }
diff --git a/target/arm/m_helper.c b/target/arm/m_helper.c
index 7a1e35ab5b6..f9a9cb466c9 100644
--- a/target/arm/m_helper.c
+++ b/target/arm/m_helper.c
@@ -2563,13 +2563,13 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t maskreg, uint32_t val)
             if (!env->v7m.secure) {
                 return;
             }
-            env->v7m.other_ss_msp = val;
+            env->v7m.other_ss_msp = val & ~3;
             return;
         case 0x89: /* PSP_NS */
             if (!env->v7m.secure) {
                 return;
             }
-            env->v7m.other_ss_psp = val;
+            env->v7m.other_ss_psp = val & ~3;
             return;
         case 0x8a: /* MSPLIM_NS */
             if (!env->v7m.secure) {
@@ -2638,6 +2638,8 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t maskreg, uint32_t val)
 
             limit = is_psp ? env->v7m.psplim[false] : env->v7m.msplim[false];
 
+            val &= ~0x3;
+
             if (val < limit) {
                 raise_exception_ra(env, EXCP_STKOF, 0, 1, GETPC());
             }
@@ -2660,16 +2662,16 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t maskreg, uint32_t val)
         break;
     case 8: /* MSP */
         if (v7m_using_psp(env)) {
-            env->v7m.other_sp = val;
+            env->v7m.other_sp = val & ~3;
         } else {
-            env->regs[13] = val;
+            env->regs[13] = val & ~3;
         }
         break;
     case 9: /* PSP */
         if (v7m_using_psp(env)) {
-            env->regs[13] = val;
+            env->regs[13] = val & ~3;
         } else {
-            env->v7m.other_sp = val;
+            env->v7m.other_sp = val & ~3;
         }
         break;
     case 10: /* MSPLIM */
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 351afa43a29..80c282669f0 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -291,6 +291,9 @@ void store_reg(DisasContext *s, int reg, TCGv_i32 var)
          */
         tcg_gen_andi_i32(var, var, s->thumb ? ~1 : ~3);
         s->base.is_jmp = DISAS_JUMP;
+    } else if (reg == 13 && arm_dc_feature(s, ARM_FEATURE_M)) {
+        /* For M-profile SP bits [1:0] are always zero */
+        tcg_gen_andi_i32(var, var, ~3);
     }
     tcg_gen_mov_i32(cpu_R[reg], var);
     tcg_temp_free_i32(var);
-- 
2.20.1