target/riscv: Add short-isa-string option

Because some operating systems don't correctly parse long ISA extension
string, this commit adds short-isa-string boolean option to disable
generating long ISA extension strings on Device Tree.

For instance, enabling Zfinx and Zdinx extensions and booting Linux (5.17 or
earlier) with FPU support caused a kernel panic.

Operating Systems which short-isa-string might be helpful:

1.  Linux (5.17 or earlier)
2.  FreeBSD (at least 14.0-CURRENT)
3.  OpenBSD (at least current development version)

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <7c1fe5f06b0a7646a47e9bcdddb1042bb60c69c8.1652181972.git.research_trasio@irq.a4lg.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -879,6 +879,8 @@ static Property riscv_cpu_properties[] = {
     DEFINE_PROP_BOOL("x-aia", RISCVCPU, cfg.aia, false),
 
     DEFINE_PROP_UINT64("resetvec", RISCVCPU, cfg.resetvec, DEFAULT_RSTVEC),
+
+    DEFINE_PROP_BOOL("short-isa-string", RISCVCPU, cfg.short_isa_string, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -1049,7 +1051,9 @@ char *riscv_isa_string(RISCVCPU *cpu)
         }
     }
     *p = '\0';
-    riscv_isa_string_ext(cpu, &isa_str, maxlen);
+    if (!cpu->cfg.short_isa_string) {
+        riscv_isa_string_ext(cpu, &isa_str, maxlen);
+    }
     return isa_str;
 }
 

target/riscv/cpu.h

@@ -425,6 +425,8 @@ struct RISCVCPUConfig {
     bool aia;
     bool debug;
     uint64_t resetvec;
+
+    bool short_isa_string;
 };
 
 typedef struct RISCVCPUConfig RISCVCPUConfig;