We don't have any form of a 'bare bones' CPU. rv64, our default CPUs,
comes with a lot of defaults. This is fine for most regular uses but
it's not suitable when more control of what is actually loaded in the
CPU is required.

A bare-bones CPU would be annoying to deal with if not by profile
support, a way to load a multitude of extensions with a single flag.
Profile support is going to be implemented shortly, so let's add a CPU
for it.

The new 'rv64i' CPU will have only RVI loaded. It is inspired in the
profile specification that dictates, for RVA22U64 [1]:

"RVA22U64 Mandatory Base
 RV64I is the mandatory base ISA for RVA22U64"

And so it seems that RV64I is the mandatory base ISA for all profiles
listed in [1], making it an ideal CPU to use with profile support.

rv64i is a CPU of type TYPE_RISCV_BARE_CPU. It has a mix of features
from pre-existent CPUs:

- it allows extensions to be enabled, like generic CPUs;
- it will not inherit extension defaults, like vendor CPUs.

This is the minimum extension set to boot OpenSBI and buildroot using
rv64i:

./build/qemu-system-riscv64 -nographic -M virt \
    -cpu rv64i,sv39=true,g=true,c=true,s=true,u=true

Our minimal riscv,isa in this case will be:

 # cat /proc/device-tree/cpus/cpu@0/riscv,isa
rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd#

[1] https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
---
 target/riscv/cpu-qom.h |  2 ++
 target/riscv/cpu.c     | 46 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/target/riscv/cpu-qom.h b/target/riscv/cpu-qom.h
index ca7dd509e3..4d1aa54311 100644
--- a/target/riscv/cpu-qom.h
+++ b/target/riscv/cpu-qom.h
@@ -24,6 +24,7 @@
 #define TYPE_RISCV_CPU "riscv-cpu"
 #define TYPE_RISCV_DYNAMIC_CPU "riscv-dynamic-cpu"
 #define TYPE_RISCV_VENDOR_CPU "riscv-vendor-cpu"
+#define TYPE_RISCV_BARE_CPU "riscv-bare-cpu"
 
 #define RISCV_CPU_TYPE_SUFFIX "-" TYPE_RISCV_CPU
 #define RISCV_CPU_TYPE_NAME(name) (name RISCV_CPU_TYPE_SUFFIX)
@@ -33,6 +34,7 @@
 #define TYPE_RISCV_CPU_BASE32           RISCV_CPU_TYPE_NAME("rv32")
 #define TYPE_RISCV_CPU_BASE64           RISCV_CPU_TYPE_NAME("rv64")
 #define TYPE_RISCV_CPU_BASE128          RISCV_CPU_TYPE_NAME("x-rv128")
+#define TYPE_RISCV_CPU_RV64I            RISCV_CPU_TYPE_NAME("rv64i")
 #define TYPE_RISCV_CPU_IBEX             RISCV_CPU_TYPE_NAME("lowrisc-ibex")
 #define TYPE_RISCV_CPU_SHAKTI_C         RISCV_CPU_TYPE_NAME("shakti-c")
 #define TYPE_RISCV_CPU_SIFIVE_E31       RISCV_CPU_TYPE_NAME("sifive-e31")
diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 220113408e..a52bf1e33c 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -370,6 +370,17 @@ static void set_satp_mode_max_supported(RISCVCPU *cpu,
 /* Set the satp mode to the max supported */
 static void set_satp_mode_default_map(RISCVCPU *cpu)
 {
+    /*
+     * Bare CPUs do not default to the max available.
+     * Users must set a valid satp_mode in the command
+     * line.
+     */
+    if (object_dynamic_cast(OBJECT(cpu), TYPE_RISCV_BARE_CPU) != NULL) {
+        warn_report("No satp mode set. Defaulting to 'bare'");
+        cpu->cfg.satp_mode.map = (1 << VM_1_10_MBARE);
+        return;
+    }
+
     cpu->cfg.satp_mode.map = cpu->cfg.satp_mode.supported;
 }
 #endif
@@ -552,6 +563,28 @@ static void rv128_base_cpu_init(Object *obj)
     set_satp_mode_max_supported(RISCV_CPU(obj), VM_1_10_SV57);
 #endif
 }
+
+static void rv64i_bare_cpu_init(Object *obj)
+{
+    CPURISCVState *env = &RISCV_CPU(obj)->env;
+    riscv_cpu_set_misa(env, MXL_RV64, RVI);
+
+    /* Remove the defaults from the parent class */
+    RISCV_CPU(obj)->cfg.ext_zicntr = false;
+    RISCV_CPU(obj)->cfg.ext_zihpm = false;
+
+    /* Set to QEMU's first supported priv version */
+    env->priv_ver = PRIV_VERSION_1_10_0;
+
+    /*
+     * Support all available satp_mode settings. The default
+     * value will be set to MBARE if the user doesn't set
+     * satp_mode manually (see set_satp_mode_default()).
+     */
+#ifndef CONFIG_USER_ONLY
+    set_satp_mode_max_supported(RISCV_CPU(obj), VM_1_10_SV64);
+#endif
+}
 #else
 static void rv32_base_cpu_init(Object *obj)
 {
@@ -1785,6 +1818,13 @@ void riscv_cpu_list(void)
         .instance_init = initfn              \
     }
 
+#define DEFINE_BARE_CPU(type_name, initfn) \
+    {                                      \
+        .name = type_name,                 \
+        .parent = TYPE_RISCV_BARE_CPU,     \
+        .instance_init = initfn            \
+    }
+
 static const TypeInfo riscv_cpu_type_infos[] = {
     {
         .name = TYPE_RISCV_CPU,
@@ -1807,6 +1847,11 @@ static const TypeInfo riscv_cpu_type_infos[] = {
         .parent = TYPE_RISCV_CPU,
         .abstract = true,
     },
+    {
+        .name = TYPE_RISCV_BARE_CPU,
+        .parent = TYPE_RISCV_CPU,
+        .abstract = true,
+    },
     DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_ANY,      riscv_any_cpu_init),
     DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_MAX,      riscv_max_cpu_init),
 #if defined(TARGET_RISCV32)
@@ -1823,6 +1868,7 @@ static const TypeInfo riscv_cpu_type_infos[] = {
     DEFINE_VENDOR_CPU(TYPE_RISCV_CPU_THEAD_C906,  rv64_thead_c906_cpu_init),
     DEFINE_VENDOR_CPU(TYPE_RISCV_CPU_VEYRON_V1,   rv64_veyron_v1_cpu_init),
     DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_BASE128,  rv128_base_cpu_init),
+    DEFINE_BARE_CPU(TYPE_RISCV_CPU_RV64I, rv64i_bare_cpu_init),
 #endif
 };
 
-- 
2.41.0