target/riscv: fix satp_mode_finalize() when satp_mode.supported = 0

In the same emulated RISC-V host, the 'host' KVM CPU takes 4 times
longer to boot than the 'rv64' KVM CPU.

The reason is an unintended behavior of riscv_cpu_satp_mode_finalize()
when satp_mode.supported = 0, i.e. when cpu_init() does not set
satp_mode_max_supported(). satp_mode_max_from_map(map) does:

31 - __builtin_clz(map)

This means that, if satp_mode.supported = 0, satp_mode_supported_max
wil be '31 - 32'. But this is C, so satp_mode_supported_max will gladly
set it to UINT_MAX (4294967295). After that, if the user didn't set a
satp_mode, set_satp_mode_default_map(cpu) will make

cfg.satp_mode.map = cfg.satp_mode.supported

So satp_mode.map = 0. And then satp_mode_map_max will be set to
satp_mode_max_from_map(cpu->cfg.satp_mode.map), i.e. also UINT_MAX. The
guard "satp_mode_map_max > satp_mode_supported_max" doesn't protect us
here since both are UINT_MAX.

And finally we have 2 loops:

        for (int i = satp_mode_map_max - 1; i >= 0; --i) {

Which are, in fact, 2 loops from UINT_MAX -1 to -1. This is where the
extra delay when booting the 'host' CPU is coming from.

Commit 43d1de3 already set a precedence for satp_mode.supported = 0
in a different manner. We're doing the same here. If supported == 0,
interpret as 'the CPU wants the OS to handle satp mode alone' and skip
satp_mode_finalize().

We'll also put a guard in satp_mode_max_from_map() to assert out if map
is 0 since the function is not ready to deal with it.

Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Fixes: 6f23aae ("riscv: Allow user to set the satp mode")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230817152903.694926-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit 3a2fc23)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target/riscv/cpu.c

@@ -299,6 +299,17 @@ static uint8_t satp_mode_from_str(const char *satp_mode_str)
 
 uint8_t satp_mode_max_from_map(uint32_t map)
 {
+    /*
+     * 'map = 0' will make us return (31 - 32), which C will
+     * happily overflow to UINT_MAX. There's no good result to
+     * return if 'map = 0' (e.g. returning 0 will be ambiguous
+     * with the result for 'map = 1').
+     *
+     * Assert out if map = 0. Callers will have to deal with
+     * it outside of this function.
+     */
+    g_assert(map > 0);
+
     /* map here has at least one bit set, so no problem with clz */
     return 31 - __builtin_clz(map);
 }
@@ -1304,9 +1315,15 @@ void riscv_cpu_validate_set_extensions(RISCVCPU *cpu, Error **errp)
 static void riscv_cpu_satp_mode_finalize(RISCVCPU *cpu, Error **errp)
 {
     bool rv32 = riscv_cpu_mxl(&cpu->env) == MXL_RV32;
-    uint8_t satp_mode_map_max;
-    uint8_t satp_mode_supported_max =
-                        satp_mode_max_from_map(cpu->cfg.satp_mode.supported);
+    uint8_t satp_mode_map_max, satp_mode_supported_max;
+
+    /* The CPU wants the OS to decide which satp mode to use */
+    if (cpu->cfg.satp_mode.supported == 0) {
+        return;
+    }
+
+    satp_mode_supported_max =
+                    satp_mode_max_from_map(cpu->cfg.satp_mode.supported);
 
     if (cpu->cfg.satp_mode.map == 0) {
         if (cpu->cfg.satp_mode.init == 0) {