target/nios2: Prevent writes to read-only or reserved control fields

Create an array of masks which detail the writable and readonly
bits for each control register.  Apply them when writing to
control registers, including the write to status during eret.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220421151735.31996-38-richard.henderson@linaro.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/target/nios2/cpu.h b/target/nios2/cpu.h
index 0027416742..da85d82faa 100644
--- a/target/nios2/cpu.h
+++ b/target/nios2/cpu.h
@@ -190,6 +190,11 @@ struct CPUArchState {
     int error_code;
 };
 
+typedef struct {
+    uint32_t writable;
+    uint32_t readonly;
+} ControlRegState;
+
 /**
  * Nios2CPU:
  * @env: #CPUNios2State
@@ -213,9 +218,17 @@ struct ArchCPU {
     uint32_t reset_addr;
     uint32_t exception_addr;
     uint32_t fast_tlb_miss_addr;
+
+    /* Bits within each control register which are reserved or readonly. */
+    ControlRegState cr_state[NUM_CR_REGS];
 };
 
 
+static inline bool nios2_cr_reserved(const ControlRegState *s)
+{
+    return (s->writable | s->readonly) == 0;
+}
+
 void nios2_tcg_init(void);
 void nios2_cpu_do_interrupt(CPUState *cs);
 void dump_mmu(CPUNios2State *env);