[PATCH v2] target/riscv: Add isa extenstion strings to the device tree

[Atish Patra]

The Linux kernel parses the ISA extensions from "riscv,isa" DT
property. It used to parse only the single letter base extensions
until now. A generic ISA extension parsing framework was proposed[1]
recently that can parse multi-letter ISA extensions as well.

Generate the extended ISA string by appending  the available ISA extensions
to the "riscv,isa" string if it is enabled so that kernel can process it.

[1] https://lkml.org/lkml/2022/2/15/263

Suggested-by: Heiko Stubner <heiko@sntech.de>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
---
Changes from v1->v2:
1. Improved the code redability by using arrays instead of individual check
---
 target/riscv/cpu.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index b0a40b83e7a8..9bf8923f164b 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -34,6 +34,13 @@
 
 /* RISC-V CPU definitions */
 
+/* This includes the null terminated character '\0' */
+#define MAX_ISA_EXT_LEN 256
+struct isa_ext_data {
+        const char *name;
+        bool enabled;
+};
+
 static const char riscv_exts[26] = "IEMAFDQCLBJTPVNSUHKORWXYZG";
 
 const char * const riscv_int_regnames[] = {
@@ -881,10 +888,35 @@ static void riscv_cpu_class_init(ObjectClass *c, void 
*data)
     device_class_set_props(dc, riscv_cpu_properties);
 }
 
+static void riscv_isa_string_ext(RISCVCPU *cpu, char *isa_str, int max_str_len)
+{
+    int offset = strnlen(isa_str, max_str_len);
+    int i;
+    struct isa_ext_data isa_edata_arr[] = {
+        { "svpbmt", cpu->cfg.ext_svpbmt   },
+        { "svinval", cpu->cfg.ext_svinval },
+        { "svnapot", cpu->cfg.ext_svnapot },
+    };
+
+    for (i = 0; i < ARRAY_SIZE(isa_edata_arr); i++) {
+        if (!isa_edata_arr[i].enabled) {
+            continue;
+        }
+        /* check available space */
+        if ((offset + strlen(isa_edata_arr[i].name) + 1) > max_str_len) {
+            qemu_log("No space left to append isa extension");
+            return;
+        }
+        offset += snprintf(isa_str + offset, max_str_len, "_%s",
+                           isa_edata_arr[i].name);
+    }
+}
+
 char *riscv_isa_string(RISCVCPU *cpu)
 {
     int i;
-    const size_t maxlen = sizeof("rv128") + sizeof(riscv_exts) + 1;
+    const size_t maxlen = sizeof("rv128") + sizeof(riscv_exts) +
+                          MAX_ISA_EXT_LEN;
     char *isa_str = g_new(char, maxlen);
     char *p = isa_str + snprintf(isa_str, maxlen, "rv%d", TARGET_LONG_BITS);
     for (i = 0; i < sizeof(riscv_exts); i++) {
@@ -893,6 +925,7 @@ char *riscv_isa_string(RISCVCPU *cpu)
         }
     }
     *p = '\0';
+    riscv_isa_string_ext(cpu, isa_str, maxlen);
     return isa_str;
 }
 
-- 
2.30.2