diff options
| author | Daniel Henrique Barboza <dbarboza@ventanamicro.com> | 2023-08-17 12:29:03 -0300 |
|---|---|---|
| committer | Alistair Francis <alistair.francis@wdc.com> | 2023-09-11 11:45:55 +1000 |
| commit | 3a2fc23563885c219c73c8f24318921daf02f3f2 (patch) | |
| tree | dd9e28255aa32f0695c95b0d986531e6d3241edc /target | |
| parent | ed67d63798f2ff31a7541ed97ac671c8e1f79297 (diff) | |
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 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>
Diffstat (limited to 'target')
| -rw-r--r-- | target/riscv/cpu.c | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c index 8071f05f15..34ac26e3ae 100644 --- a/target/riscv/cpu.c +++ b/target/riscv/cpu.c @@ -309,6 +309,17 @@ 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); } @@ -1333,9 +1344,15 @@ 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) { |