target/riscv: Emulate TIME CSRs for privileged mode

Currently, TIME CSRs are emulated only for user-only mode. This
patch add TIME CSRs emulation for privileged mode.

For privileged mode, the TIME CSRs will return value provided
by rdtime callback which is registered by QEMU machine/platform
emulation (i.e. CLINT emulation). If rdtime callback is not
available then the monitor (i.e. OpenSBI) will trap-n-emulate
TIME CSRs in software.

We see 25+% performance improvement in hackbench numbers when
TIME CSRs are not trap-n-emulated.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

target/riscv/cpu.h

@@ -159,6 +159,7 @@ struct CPURISCVState {
     target_ulong htval;
     target_ulong htinst;
     target_ulong hgatp;
+    uint64_t htimedelta;
 
     /* Virtual CSRs */
     target_ulong vsstatus;
@@ -201,6 +202,9 @@ struct CPURISCVState {
     /* physical memory protection */
     pmp_table_t pmp_state;
 
+    /* machine specific rdtime callback */
+    uint64_t (*rdtime_fn)(void);
+
     /* True if in debugger mode.  */
     bool debugger;
 #endif
@@ -322,6 +326,7 @@ void riscv_cpu_swap_hypervisor_regs(CPURISCVState *env);
 int riscv_cpu_claim_interrupts(RISCVCPU *cpu, uint32_t interrupts);
 uint32_t riscv_cpu_update_mip(RISCVCPU *cpu, uint32_t mask, uint32_t value);
 #define BOOL_TO_MASK(x) (-!!(x)) /* helper for riscv_cpu_update_mip value */
+void riscv_cpu_set_rdtime_fn(CPURISCVState *env, uint64_t (*fn)(void));
 #endif
 void riscv_cpu_set_mode(CPURISCVState *env, target_ulong newpriv);
 

target/riscv/cpu_helper.c

@@ -258,6 +258,11 @@ uint32_t riscv_cpu_update_mip(RISCVCPU *cpu, uint32_t mask, uint32_t value)
     return old;
 }
 
+void riscv_cpu_set_rdtime_fn(CPURISCVState *env, uint64_t (*fn)(void))
+{
+    env->rdtime_fn = fn;
+}
+
 void riscv_cpu_set_mode(CPURISCVState *env, target_ulong newpriv)
 {
     if (newpriv > PRV_M) {

target/riscv/csr.c

@@ -238,6 +238,32 @@ static int read_timeh(CPURISCVState *env, int csrno, target_ulong *val)
 
 #else /* CONFIG_USER_ONLY */
 
+static int read_time(CPURISCVState *env, int csrno, target_ulong *val)
+{
+    uint64_t delta = riscv_cpu_virt_enabled(env) ? env->htimedelta : 0;
+
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+    *val = env->rdtime_fn() + delta;
+    return 0;
+}
+
+#if defined(TARGET_RISCV32)
+static int read_timeh(CPURISCVState *env, int csrno, target_ulong *val)
+{
+    uint64_t delta = riscv_cpu_virt_enabled(env) ? env->htimedelta : 0;
+
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+    *val = (env->rdtime_fn() + delta) >> 32;
+    return 0;
+}
+#endif
+
 /* Machine constants */
 
 #define M_MODE_INTERRUPTS  (MIP_MSIP | MIP_MTIP | MIP_MEIP)
@@ -930,6 +956,56 @@ static int write_hgatp(CPURISCVState *env, int csrno, target_ulong val)
     return 0;
 }
 
+static int read_htimedelta(CPURISCVState *env, int csrno, target_ulong *val)
+{
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+#if defined(TARGET_RISCV32)
+    *val = env->htimedelta & 0xffffffff;
+#else
+    *val = env->htimedelta;
+#endif
+    return 0;
+}
+
+static int write_htimedelta(CPURISCVState *env, int csrno, target_ulong val)
+{
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+#if defined(TARGET_RISCV32)
+    env->htimedelta = deposit64(env->htimedelta, 0, 32, (uint64_t)val);
+#else
+    env->htimedelta = val;
+#endif
+    return 0;
+}
+
+#if defined(TARGET_RISCV32)
+static int read_htimedeltah(CPURISCVState *env, int csrno, target_ulong *val)
+{
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+    *val = env->htimedelta >> 32;
+    return 0;
+}
+
+static int write_htimedeltah(CPURISCVState *env, int csrno, target_ulong val)
+{
+    if (!env->rdtime_fn) {
+        return -1;
+    }
+
+    env->htimedelta = deposit64(env->htimedelta, 32, 32, (uint64_t)val);
+    return 0;
+}
+#endif
+
 /* Virtual CSR Registers */
 static int read_vsstatus(CPURISCVState *env, int csrno, target_ulong *val)
 {
@@ -1202,14 +1278,12 @@ static riscv_csr_operations csr_ops[CSR_TABLE_SIZE] = {
     [CSR_INSTRETH] =            { ctr,  read_instreth                       },
 #endif
 
-    /* User-level time CSRs are only available in linux-user
+    /* In privileged mode, the monitor will have to emulate TIME CSRs only if
-     * In privileged mode, the monitor emulates these CSRs */
+     * rdtime callback is not provided by machine/platform emulation */
-#if defined(CONFIG_USER_ONLY)
     [CSR_TIME] =                { ctr,  read_time                           },
 #if defined(TARGET_RISCV32)
     [CSR_TIMEH] =               { ctr,  read_timeh                          },
 #endif
-#endif
 
 #if !defined(CONFIG_USER_ONLY)
     /* Machine Timers and Counters */
@@ -1275,6 +1349,10 @@ static riscv_csr_operations csr_ops[CSR_TABLE_SIZE] = {
     [CSR_HTVAL] =               { hmode,   read_htval,       write_htval      },
     [CSR_HTINST] =              { hmode,   read_htinst,      write_htinst     },
     [CSR_HGATP] =               { hmode,   read_hgatp,       write_hgatp      },
+    [CSR_HTIMEDELTA] =          { hmode,   read_htimedelta,  write_htimedelta },
+#if defined(TARGET_RISCV32)
+    [CSR_HTIMEDELTAH] =         { hmode,   read_htimedeltah, write_htimedeltah},
+#endif
 
     [CSR_VSSTATUS] =            { hmode,   read_vsstatus,    write_vsstatus   },
     [CSR_VSIP] =                { hmode,   NULL,     NULL,     rmw_vsip       },