RISC-V: Clear load reservations on context switch and SC

This prevents a load reservation from being placed in one context/process,
then being used in another, resulting in an SC succeeding incorrectly and
breaking atomics.

Backports commit c13b169f1a3dd158d6c75727cdc388f95988db39 from qemu

qemu/target/riscv/cpu.c

@@ -295,6 +295,7 @@ static void riscv_cpu_reset(CPUState *cs)
     env->pc = env->resetvec;
 #endif
     cs->exception_index = EXCP_NONE;
+    env->load_res = -1;
     set_default_nan_mode(1, &env->fp_status);
 }
 

qemu/target/riscv/cpu_helper.c

@@ -114,6 +114,16 @@ void riscv_cpu_set_mode(CPURISCVState *env, target_ulong newpriv)
     }
     /* tlb_flush is unnecessary as mode is contained in mmu_idx */
     env->priv = newpriv;
+
+    /*
+     * Clear the load reservation - otherwise a reservation placed in one
+     * context/process can be used by another, resulting in an SC succeeding
+     * incorrectly. Version 2.2 of the ISA specification explicitly requires
+     * this behaviour, while later revisions say that the kernel "should" use
+     * an SC instruction to force the yielding of a load reservation on a
+     * preemptive context switch. As a result, do both.
+     */
+    env->load_res = -1;
 }
 
 /* get_physical_address - get the physical address for this virtual address

qemu/target/riscv/insn_trans/trans_rva.inc.c

@@ -63,7 +63,7 @@ static inline bool gen_sc(DisasContext *ctx, arg_atomic *a, TCGMemOp mop)
 
     gen_set_label(tcg_ctx, l1);
     /*
-     * Address comparion failure.  However, we still need to
+     * Address comparison failure.  However, we still need to
      * provide the memory barrier implied by AQ/RL.
      */
     tcg_gen_mb(tcg_ctx, TCG_MO_ALL + a->aq * TCG_BAR_LDAQ + a->rl * TCG_BAR_STRL);
@@ -71,6 +71,12 @@ static inline bool gen_sc(DisasContext *ctx, arg_atomic *a, TCGMemOp mop)
     gen_set_gpr(ctx, a->rd, dat);
 
     gen_set_label(tcg_ctx, l2);
+    /*
+     * Clear the load reservation, since an SC must fail if there is
+     * an SC to any address, in between an LR and SC pair.
+     */
+    tcg_gen_movi_tl(tcg_ctx, tcg_ctx->load_res_risc, -1);
+
     tcg_temp_free(tcg_ctx, dat);
     tcg_temp_free(tcg_ctx, src1);
     tcg_temp_free(tcg_ctx, src2);