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
43d1de32f8 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:
6f23aaeb9b ("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>
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);
}
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) {