[PATCH for-8.1 v4 23/25] target/riscv: rework write_misa()

[Daniel Henrique Barboza <dbarboza@ventanamicro.com>]

write_misa() must use as much common logic as possible. We want to open
code just the bits that are exclusive to the CSR write operation and TCG
internals.

Rewrite write_misa() to work as follows:

- mask the write using misa_ext_mask to avoid enabling unsupported
  extensions;

- suppress RVC if the next insn isn't aligned;

- handle RVE. This is done by filtering all bits but RVE from 'val'.
  Setting RVE will forcefully set only RVE - assuming it gets
  validated afterwards;

- emulate the steps done by realize(): validate the candidate misa_ext
  val, then validate the configuration with the candidate misa_ext val,
  and finally commit the changes to cpu->cfg.

If any of the validation steps fails, the write operation is a no-op.

Let's keep write_misa() as experimental for now until this logic gains
enough mileage.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/cpu.c | 12 ++++------
 target/riscv/cpu.h |  6 +++++
 target/riscv/csr.c | 59 ++++++++++++++++++++++++++--------------------
 3 files changed, 45 insertions(+), 32 deletions(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 0e6b8fb45e..41b17ba0c3 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -1018,9 +1018,8 @@ static void riscv_cpu_disable_priv_spec_isa_exts(RISCVCPU *cpu)
 }
 
 
-static void riscv_cpu_validate_misa_ext(CPURISCVState *env,
-                                        uint32_t misa_ext,
-                                        Error **errp)
+void riscv_cpu_validate_misa_ext(CPURISCVState *env, uint32_t misa_ext,
+                                 Error **errp)
 {
     Error *local_err = NULL;
 
@@ -1113,9 +1112,8 @@ static void riscv_cpu_validate_misa_mxl(RISCVCPU *cpu, Error **errp)
  * candidate misa_ext value. No changes in env->misa_ext
  * are made.
  */
-static void riscv_cpu_validate_extensions(RISCVCPU *cpu,
-                                          uint32_t misa_ext,
-                                          Error **errp)
+void riscv_cpu_validate_extensions(RISCVCPU *cpu, uint32_t misa_ext,
+                                   Error **errp)
 {
     if (cpu->cfg.epmp && !cpu->cfg.pmp) {
         /*
@@ -1206,7 +1204,7 @@ static void riscv_cpu_validate_extensions(RISCVCPU *cpu,
     }
 }
 
-static void riscv_cpu_commit_cpu_cfg(RISCVCPU *cpu)
+void riscv_cpu_commit_cpu_cfg(RISCVCPU *cpu)
 {
     if (cpu->cfg.ext_zk) {
         cpu->cfg.ext_zkn = true;
diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index dbb4df9df0..ca2ba6a647 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -593,6 +593,12 @@ bool riscv_cpu_tlb_fill(CPUState *cs, vaddr address, int size,
 char *riscv_isa_string(RISCVCPU *cpu);
 void riscv_cpu_list(void);
 
+void riscv_cpu_validate_misa_ext(CPURISCVState *env, uint32_t misa_ext,
+                                 Error **errp);
+void riscv_cpu_validate_extensions(RISCVCPU *cpu, uint32_t misa_ext,
+                                   Error **errp);
+void riscv_cpu_commit_cpu_cfg(RISCVCPU *cpu);
+
 #define cpu_list riscv_cpu_list
 #define cpu_mmu_index riscv_cpu_mmu_index
 
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index d522efc0b6..8d5e8f9ad1 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1343,39 +1343,17 @@ static RISCVException read_misa(CPURISCVState *env, int csrno,
 static RISCVException write_misa(CPURISCVState *env, int csrno,
                                  target_ulong val)
 {
+    RISCVCPU *cpu = env_archcpu(env);
+    Error *local_err = NULL;
+
     if (!riscv_cpu_cfg(env)->misa_w) {
         /* drop write to misa */
         return RISCV_EXCP_NONE;
     }
 
-    /* 'I' or 'E' must be present */
-    if (!(val & (RVI | RVE))) {
-        /* It is not, drop write to misa */
-        return RISCV_EXCP_NONE;
-    }
-
-    /* 'E' excludes all other extensions */
-    if (val & RVE) {
-        /*
-         * when we support 'E' we can do "val = RVE;" however
-         * for now we just drop writes if 'E' is present.
-         */
-        return RISCV_EXCP_NONE;
-    }
-
-    /*
-     * misa.MXL writes are not supported by QEMU.
-     * Drop writes to those bits.
-     */
-
     /* Mask extensions that are not supported by this hart */
     val &= env->misa_ext_mask;
 
-    /* 'D' depends on 'F', so clear 'D' if 'F' is not present */
-    if ((val & RVD) && !(val & RVF)) {
-        val &= ~RVD;
-    }
-
     /*
      * Suppress 'C' if next instruction is not aligned
      * TODO: this should check next_pc
@@ -1389,6 +1367,37 @@ static RISCVException write_misa(CPURISCVState *env, int csrno,
         return RISCV_EXCP_NONE;
     }
 
+    /*
+     * We'll handle special cases in separate. If one
+     * of these bits are enabled we'll handle them and
+     * end the CSR write.
+     */
+    if (val & RVE && !(env->misa_ext & RVE)) {
+        /*
+         * RVE must be enabled by itself. Clear all other
+         * misa_env bits and let the validation do its
+         * job.
+         */
+        val &= RVE;
+    }
+
+    /*
+     * This flow is similar to what riscv_cpu_realize() does,
+     * with the difference that we will update env->misa_ext
+     * value if everything is ok.
+     */
+    riscv_cpu_validate_misa_ext(env, val, &local_err);
+    if (local_err != NULL) {
+        return RISCV_EXCP_NONE;
+    }
+
+    riscv_cpu_validate_extensions(cpu, val, &local_err);
+    if (local_err != NULL) {
+        return RISCV_EXCP_NONE;
+    }
+
+    riscv_cpu_commit_cpu_cfg(cpu);
+
     if (!(val & RVF)) {
         env->mstatus &= ~MSTATUS_FS;
     }
-- 
2.39.2