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[189.69.160.189]) by smtp.gmail.com with ESMTPSA id a24-20020a05680804d800b003a5b027ccb2sm7606394oie.38.2023.08.17.08.29.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Aug 2023 08:29:08 -0700 (PDT) From: Daniel Henrique Barboza To: qemu-devel@nongnu.org Cc: qemu-riscv@nongnu.org, alistair.francis@wdc.com, bmeng@tinylab.org, liweiwei@iscas.ac.cn, zhiwei_liu@linux.alibaba.com, palmer@rivosinc.com, Daniel Henrique Barboza , Alexandre Ghiti Subject: [PATCH] target/riscv: fix satp_mode_finalize() when satp_mode.supported = 0 Date: Thu, 17 Aug 2023 12:29:03 -0300 Message-ID: <20230817152903.694926-1-dbarboza@ventanamicro.com> X-Mailer: git-send-email 2.41.0 MIME-Version: 1.0 Received-SPF: pass client-ip=2607:f8b0:4864:20::235; envelope-from=dbarboza@ventanamicro.com; helo=mail-oi1-x235.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org 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 Fixes: 6f23aaeb9b ("riscv: Allow user to set the satp mode") Signed-off-by: Daniel Henrique Barboza Reviewed-by: Andrew Jones --- target/riscv/cpu.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c index d608026a28..86da93c7bc 100644 --- a/target/riscv/cpu.c +++ b/target/riscv/cpu.c @@ -349,6 +349,17 @@ 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); } @@ -1387,9 +1398,15 @@ 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) {