diff --git a/articles/20230816-riscv-isa-discovery-4-opensbi.md b/articles/20230816-riscv-isa-discovery-4-opensbi.md new file mode 100644 index 0000000000000000000000000000000000000000..06eca9572f5b97cf6f83c7e2d4c255b3e6061331 --- /dev/null +++ b/articles/20230816-riscv-isa-discovery-4-opensbi.md @@ -0,0 +1,422 @@ +> Corrector: [TinyCorrect](https://gitee.com/tinylab/tinycorrect) v0.2-rc2 - [spaces quotes header comments urls]`
` +> Author: YJMSTR [jay1273062855@outlook.com](mailto:jay1273062855@outlook.com)`
` +> Date: 2023/08/16`
` +> Revisor: Bin Meng, Falcon`
` +> Project: [RISC-V Linux 内核剖析](https://gitee.com/tinylab/riscv-linux)`
` +> Sponsor: PLCT Lab, ISCAS + +# OpenSBI RISC-V ISA 扩展检测与支持方式分析 + +## 前言 + +本文是 RISC-V 扩展的软硬件支持方式调研系列的第 4 篇文章，将以 OpenSBI 为例分析 SBI 对 RISC-V 扩展的检测与支持方式。 + +SBI 指 Supervisor Binary Interface，其将 M 模式下对硬件的操作抽象成 S 模式软件可以调用的统一接口，来将硬件与 S 模式软件进行一定程度的解耦；此外，SBI 也是 HS 模式的虚拟机与 VS 模式程序之间的推荐接口。RISC-V 定义了 [SBI 的规范][001]，SBI 规范中并未指定任何的硬件检测机制，S 模式的软件必须通过诸如 Device Tree 或 ACPI 这样的机制来实现硬件检测。 + +本文基于 QEMU v8.1.0-rc0 与 OpenSBI v1.3.1 进行分析。 + +## OpenSBI 快速上手 + +之前社区已有介绍 OpenSBI 的文章（参考：[OpenSBI 快速上手][002]），此处不再赘述。 + +目前 OpenSBI 的最新 release 版本是 OpenSBI v1.3.1，按如下命令获得源码并编译。 + +```sh +$ git clone https://github.com/riscv-software-src/opensbi.git +$ cd opensbi +$ git checkout v1.3.1 +$ export CROSS_COMPILE=riscv64-linux-gnu- +$ make all PLATFORM=generic PLATFORM_RISCV_XLEN=64 +``` + +其中 PLATFORM 选择 QEMU RISC-V 'virt' 虚拟板卡所需的 generic 平台。 + +在 `qemu-system-riscv64` 启动时通过 `-bios` 参数指定刚才编译出的 OpenSBI firmware，可以得到如下启动日志： + +```sh +$ qemu-system-riscv64 -M virt -m 256M -nographic -bios build/platform/generic/firmware/fw_jump.bin + +OpenSBI v1.3.1 + ____ _____ ____ _____ + / __ \ / ____| _ \_ _| + | | | |_ __ ___ _ __ | (___ | |_) || | + | | | | '_ \ / _ \ '_ \ \___ \| _ < | | + | |__| | |_) | __/ | | |____) | |_) || |_ + \____/| .__/ \___|_| |_|_____/|___/_____| + | | + |_| + +Platform Name : riscv-virtio,qemu +Platform Features : medeleg +Platform HART Count : 1 +Platform IPI Device : aclint-mswi +Platform Timer Device : aclint-mtimer @ 10000000Hz +Platform Console Device : uart8250 +Platform HSM Device : --- +Platform PMU Device : --- +Platform Reboot Device : sifive_test +Platform Shutdown Device : sifive_test +Platform Suspend Device : --- +Platform CPPC Device : --- +Firmware Base : 0x80000000 +Firmware Size : 194 KB +Firmware RW Offset : 0x20000 +Firmware RW Size : 66 KB +Firmware Heap Offset : 0x28000 +Firmware Heap Size : 34 KB (total), 2 KB (reserved), 9 KB (used), 22 KB (free) +Firmware Scratch Size : 4096 B (total), 760 B (used), 3336 B (free) +Runtime SBI Version : 1.0 + +Domain0 Name : root +Domain0 Boot HART : 0 +Domain0 HARTs : 0* +Domain0 Region00 : 0x0000000002000000-0x000000000200ffff M: (I,R,W) S/U: () +Domain0 Region01 : 0x0000000080000000-0x000000008001ffff M: (R,X) S/U: () +Domain0 Region02 : 0x0000000080020000-0x000000008003ffff M: (R,W) S/U: () +Domain0 Region03 : 0x0000000000000000-0xffffffffffffffff M: (R,W,X) S/U: (R,W,X) +Domain0 Next Address : 0x0000000080200000 +Domain0 Next Arg1 : 0x0000000082200000 +Domain0 Next Mode : S-mode +Domain0 SysReset : yes +Domain0 SysSuspend : yes + +Boot HART ID : 0 +Boot HART Domain : root +Boot HART Priv Version : v1.12 +Boot HART Base ISA : rv64imafdch +Boot HART ISA Extensions : time,sstc +Boot HART PMP Count : 16 +Boot HART PMP Granularity : 4 +Boot HART PMP Address Bits: 54 +Boot HART MHPM Count : 16 +Boot HART MIDELEG : 0x0000000000001666 +Boot HART MEDELEG : 0x0000000000f0b509 + +``` + + 从中可以看见 Boot HART Base ISA 信息与 Boot HART ISA Extensions 信息。 + +QEMU RISC-V 平台内置了 OpenSBI，如果在启动参数中将 `-bios` 设置为 default 或不指定该参数，就会启动内置的 OpenSBI。 + +```sh +$ qemu-system-riscv64 -M virt -m 256M -nographic + +OpenSBI v1.3 + ____ _____ ____ _____ + / __ \ / ____| _ \_ _| + | | | |_ __ ___ _ __ | (___ | |_) || | + | | | | '_ \ / _ \ '_ \ \___ \| _ < | | + | |__| | |_) | __/ | | |____) | |_) || |_ + \____/| .__/ \___|_| |_|_____/|___/_____| + | | + |_| + +Platform Name : riscv-virtio,qemu +Platform Features : medeleg +Platform HART Count : 1 +Platform IPI Device : aclint-mswi +Platform Timer Device : aclint-mtimer @ 10000000Hz +Platform Console Device : uart8250 +Platform HSM Device : --- +Platform PMU Device : --- +Platform Reboot Device : sifive_test +Platform Shutdown Device : sifive_test +Platform Suspend Device : --- +Platform CPPC Device : --- +Firmware Base : 0x80000000 +Firmware Size : 194 KB +Firmware RW Offset : 0x20000 +Firmware RW Size : 66 KB +Firmware Heap Offset : 0x28000 +Firmware Heap Size : 34 KB (total), 2 KB (reserved), 9 KB (used), 22 KB (free) +Firmware Scratch Size : 4096 B (total), 760 B (used), 3336 B (free) +Runtime SBI Version : 1.0 + +Domain0 Name : root +Domain0 Boot HART : 0 +Domain0 HARTs : 0* +Domain0 Region00 : 0x0000000002000000-0x000000000200ffff M: (I,R,W) S/U: () +Domain0 Region01 : 0x0000000080000000-0x000000008001ffff M: (R,X) S/U: () +Domain0 Region02 : 0x0000000080020000-0x000000008003ffff M: (R,W) S/U: () +Domain0 Region03 : 0x0000000000000000-0xffffffffffffffff M: (R,W,X) S/U: (R,W,X) +Domain0 Next Address : 0x0000000000000000 +Domain0 Next Arg1 : 0x000000008fe00000 +Domain0 Next Mode : S-mode +Domain0 SysReset : yes +Domain0 SysSuspend : yes + +Boot HART ID : 0 +Boot HART Domain : root +Boot HART Priv Version : v1.12 +Boot HART Base ISA : rv64imafdch +Boot HART ISA Extensions : time,sstc +Boot HART PMP Count : 16 +Boot HART PMP Granularity : 4 +Boot HART PMP Address Bits: 54 +Boot HART MHPM Count : 16 +Boot HART MIDELEG : 0x0000000000001666 +Boot HART MEDELEG : 0x0000000000f0b509 + +``` + +可以看见，目前 QEMU v8.1.0-rc0 内置的 OpenSBI 版本为 v1.3。 + +## 源码分析 + +在 OpenSBI 的 `docs/platform/qemu_virt.md` 中有使用 GDB 对 QEMU + OpenSBI 进行调试的教程，此处不再赘述。 + +前一小节中 OpenSBI 启动时会在命令行中输出当前设备的相关信息，其中输出扩展信息的相关代码如下： + +```c +/* lib/sbi/sbi_init.c:152 */ + +static void sbi_boot_print_hart(struct sbi_scratch *scratch, u32 hartid) +{ + int xlen; + char str[128]; + const struct sbi_domain *dom = sbi_domain_thishart_ptr(); + + if (scratch->options & SBI_SCRATCH_NO_BOOT_PRINTS) + return; + + /* Determine MISA XLEN and MISA string */ + xlen = misa_xlen(); + if (xlen < 1) { + sbi_printf("Error %d getting MISA XLEN\n", xlen); + sbi_hart_hang(); + } + + /* Boot HART details */ + sbi_printf("Boot HART ID : %u\n", hartid); + sbi_printf("Boot HART Domain : %s\n", dom->name); + sbi_hart_get_priv_version_str(scratch, str, sizeof(str)); + sbi_printf("Boot HART Priv Version : %s\n", str); + misa_string(xlen, str, sizeof(str)); + sbi_printf("Boot HART Base ISA : %s\n", str); + sbi_hart_get_extensions_str(scratch, str, sizeof(str)); + sbi_printf("Boot HART ISA Extensions : %s\n", str); + sbi_printf("Boot HART PMP Count : %d\n", + sbi_hart_pmp_count(scratch)); + sbi_printf("Boot HART PMP Granularity : %lu\n", + sbi_hart_pmp_granularity(scratch)); + sbi_printf("Boot HART PMP Address Bits: %d\n", + sbi_hart_pmp_addrbits(scratch)); + sbi_printf("Boot HART MHPM Count : %d\n", + sbi_hart_mhpm_count(scratch)); + sbi_hart_delegation_dump(scratch, "Boot HART ", " "); +} +``` + +可以看见上述代码分别调用了 `misa_string` 和 `sbi_hart_get_extensions_str` 函数来获取 Boot HART Base ISA 信息与 Boot HART ISA Extensions 信息。 + +`misa_string` 的实现如下： + +```c +/* lib/sbi/riscv_asm.c:53 */ + +void misa_string(int xlen, char *out, unsigned int out_sz) +{ + unsigned int i, pos = 0; + const char valid_isa_order[] = "iemafdqclbjtpvnhkorwxyzg"; + + if (!out) + return; + + if (5 <= (out_sz - pos)) { + out[pos++] = 'r'; + out[pos++] = 'v'; + switch (xlen) { + case 1: + out[pos++] = '3'; + out[pos++] = '2'; + break; + case 2: + out[pos++] = '6'; + out[pos++] = '4'; + break; + case 3: + out[pos++] = '1'; + out[pos++] = '2'; + out[pos++] = '8'; + break; + default: + sbi_panic("%s: Unknown misa.MXL encoding %d", + __func__, xlen); + return; + } + } + + for (i = 0; i < array_size(valid_isa_order) && (pos < out_sz); i++) { + if (misa_extension_imp(valid_isa_order[i])) + out[pos++] = valid_isa_order[i]; + } + + if (pos < out_sz) + out[pos++] = '\0'; +} +``` + +上述代码通过枚举 misa 中每一个扩展所对应的字符，作为参数传入 `misa_extension_imp` 函数来判断该扩展是否启用，若支持该扩展，返回非 0 值： + +```c +/* lib/sbi/riscv_asm.c:16 */ + +/* determine CPU extension, return non-zero support */ +int misa_extension_imp(char ext) +{ + unsigned long misa = csr_read(CSR_MISA); + + if (misa) { + if ('A' <= ext && ext <= 'Z') + return misa & (1 << (ext - 'A')); + if ('a' <= ext && ext <= 'z') + return misa & (1 << (ext - 'a')); + return 0; + } + + return sbi_platform_misa_extension(sbi_platform_thishart_ptr(), ext); +} +``` + +其中，`csr_read` 函数通过内联汇编读取 misa 寄存器的值。如果读出的 misa 值是 0，说明当前设备没有实现 MISA 寄存器，需要通过其它非标准的方式来确定启用了哪些扩展。这里首先通过 `sbi_platform_thishart_ptr` 函数从 mscratch 寄存器中读出当前 hart 的 `struct sbi_platform` 结构体，该结构体中包含了 `struct sbi_platform_operations` 结构体的所在地址，而这个结构体中又包括了平台特定的 `misa_check_extension` 函数，`sbi_platform_misa_extension` 会判断平台是否实现了该函数，如果实现了该函数，就调用平台所指定的函数进行判断： + +```c +/* include/sbi/sbi_platform.h:461 */ + +/** + * Check CPU extension in MISA + * + * @param plat pointer to struct sbi_platform + * @param ext shorthand letter for CPU extensions + * + * @return zero for not-supported and non-zero for supported + */ +static inline int sbi_platform_misa_extension(const struct sbi_platform *plat, + char ext) +{ + if (plat && sbi_platform_ops(plat)->misa_check_extension) + return sbi_platform_ops(plat)->misa_check_extension(ext); + return 0; +} +``` + +回到 `sbi_boot_print_hart` 函数，对于不在 misa 中的扩展，OpenSBI 通过 `sbi_hart_get_extensions_str` 函数进行检测，该函数如下： + +```c +/* lib/sbi/sbi_hart.c:463 */ + +/** + * Get the hart extensions in string format + * + * @param scratch pointer to the HART scratch space + * @param extensions_str pointer to a char array where the extensions string + * will be updated + * @param nestr length of the features_str. The feature string will be + * truncated if nestr is not long enough. + */ +void sbi_hart_get_extensions_str(struct sbi_scratch *scratch, + char *extensions_str, int nestr) +{ + struct sbi_hart_features *hfeatures = + sbi_scratch_offset_ptr(scratch, hart_features_offset); + int offset = 0, ext = 0; + char *temp; + + if (!extensions_str || nestr <= 0) + return; + sbi_memset(extensions_str, 0, nestr); + + if (!hfeatures->extensions) + goto done; + + do { + if (hfeatures->extensions & BIT(ext)) { + temp = sbi_hart_extension_id2string(ext); + if (temp) { + sbi_snprintf(extensions_str + offset, + nestr - offset, + "%s,", temp); + offset = offset + sbi_strlen(temp) + 1; + } + } + + ext++; + } while (ext < SBI_HART_EXT_MAX); + +done: + if (offset) + extensions_str[offset - 1] = '\0'; + else + sbi_strncpy(extensions_str, "none", nestr); +} +``` + +`hfeatures` 结构体中包含一个 `unsigned long extensions` 成员，其每一位对应一种扩展，目前仅支持下述几种 misa 寄存器无法标识的扩展： + +```c +/* include/sbi/sbi_hart.h:27 */ + +/** Possible ISA extensions of a hart */ +enum sbi_hart_extensions { + /** Hart has Sscofpmt extension */ + SBI_HART_EXT_SSCOFPMF = 0, + /** HART has HW time CSR (extension name not available) */ + SBI_HART_EXT_TIME, + /** HART has AIA M-mode CSRs */ + SBI_HART_EXT_SMAIA, + /** HART has Smstateen CSR **/ + SBI_HART_EXT_SMSTATEEN, + /** HART has Sstc extension */ + SBI_HART_EXT_SSTC, + + /** Maximum index of Hart extension */ + SBI_HART_EXT_MAX, +}; +``` + +这些非 misa 扩展会组成为 `extensions_str` 字符串，在 OpenSBI 的启动日志中作为 `Boot HART ISA Extensions` 输出。 + +本文所分析的 OpenSBI 版本虽然是当前最新的 Release 版本，但就在近几天内 OpenSBI 的开发版本中又新增了对若干扩展的支持，最新的 `sbi_hart_extensions` 如下所示： + +```c +/* include/sbi/sbi_hart.h:27 */ + +/** Possible ISA extensions of a hart */ +enum sbi_hart_extensions { + /** HART has AIA M-mode CSRs */ + SBI_HART_EXT_SMAIA = 0, + /** HART has Smepmp */ + SBI_HART_EXT_SMEPMP, + /** HART has Smstateen CSR **/ + SBI_HART_EXT_SMSTATEEN, + /** Hart has Sscofpmt extension */ + SBI_HART_EXT_SSCOFPMF, + /** HART has Sstc extension */ + SBI_HART_EXT_SSTC, + /** HART has Zicntr extension (i.e. HW cycle, time & instret CSRs) */ + SBI_HART_EXT_ZICNTR, + /** HART has Zihpm extension */ + SBI_HART_EXT_ZIHPM, + /** Hart has Smcntrpmf extension */ + SBI_HART_EXT_SMCNTRPMF, + + /** Maximum index of Hart extension */ + SBI_HART_EXT_MAX, +}; +``` + +## 总结 + +本文对 OpenSBI 目前的 RISC-V ISA 扩展检测机制与支持情况进行了简要分析，SBI 规范并未规定 SBI 检测硬件的方式，S 模式的软件必须通过诸如 Device Tree 或 ACPI 这样的机制来实现硬件检测。不过 OpenSBI 会在启动时进行 RISC-V ISA 扩展的检测，并在启动时输出当前 hart 支持的部分扩展，这些扩展被 OpenSBI 分为 Base ISA（misa 中包含的扩展）和 ISA Extension 分别进行检测和输出。 + +Base ISA 的检测是通过直接读取 misa CSR 来完成的，当未实现 misa 寄存器时，则是通过读取 mscratch 寄存器来获取平台特定的检测函数，调用该函数进行检测。 + +ISA Extension 通过读取 mscratch 寄存器获得 hfeature 结构体的地址，从该结构体中判断实现了哪些扩展。目前 OpenSBI 启动时仅会通过这种方式检测少数几种扩展，如 SMAIA 等。 + +## 参考资料 + +- [riscv-sbi-doc][001] +- [OpenSBI 快速上手][002] + +[001]: https://github.com/riscv-non-isa/riscv-sbi-doc +[002]: https://tinylab.org/riscv-opensbi-quickstart/