Am Dienstag, 14. März 2023, 19:32:14 CET schrieb Evan Green:
There's been a bunch of off-list discussions about this, including at Plumbers. The original plan was to do something involving providing an ISA string to userspace, but ISA strings just aren't sufficient for a stable ABI any more: in order to parse an ISA string users need the version of the specifications that the string is written to, the version of each extension (sometimes at a finer granularity than the RISC-V releases/versions encode), and the expected use case for the ISA string (ie, is it a U-mode or M-mode string). That's a lot of complexity to try and keep ABI compatible and it's probably going to continue to grow, as even if there's no more complexity in the specifications we'll have to deal with the various ISA string parsing oddities that end up all over userspace.
Instead this patch set takes a very different approach and provides a set of key/value pairs that encode various bits about the system. The big advantage here is that we can clearly define what these mean so we can ensure ABI stability, but it also allows us to encode information that's unlikely to ever appear in an ISA string (see the misaligned access performance, for example). The resulting interface looks a lot like what arm64 and x86 do, and will hopefully fit well into something like ACPI in the future.
The actual user interface is a syscall, with a vDSO function in front of it. The vDSO function can answer some queries without a syscall at all, and falls back to the syscall for cases it doesn't have answers to. Currently we prepopulate it with an array of answers for all keys and a CPU set of "all CPUs". This can be adjusted as necessary to provide fast answers to the most common queries.
I've built myself a small test-program [see below], to check the feature on the d1-nezha board. Which is how I found the tiny c-extension issue.
Series works as expected there, so patches 1-4 on a d1-nezha:
Tested-by: Heiko Stuebner heiko.stuebner@vrull.eu
hwprobe.c: ---------------- #include <linux/types.h> #include <sys/syscall.h> #include <stdio.h> #include <unistd.h>
#define __NR_riscv_hwprobe 258
struct riscv_hwprobe { __s64 key; __u64 value; };
#define RISCV_HWPROBE_KEY_MVENDORID 0 #define RISCV_HWPROBE_KEY_MARCHID 1 #define RISCV_HWPROBE_KEY_MIMPID 2 #define RISCV_HWPROBE_KEY_BASE_BEHAVIOR 3 #define RISCV_HWPROBE_BASE_BEHAVIOR_IMA (1 << 0) #define RISCV_HWPROBE_KEY_IMA_EXT_0 4 #define RISCV_HWPROBE_IMA_FD (1 << 0) #define RISCV_HWPROBE_IMA_C (1 << 1) #define RISCV_HWPROBE_KEY_CPUPERF_0 5 #define RISCV_HWPROBE_MISALIGNED_UNKNOWN (0 << 0) #define RISCV_HWPROBE_MISALIGNED_EMULATED (1 << 0) #define RISCV_HWPROBE_MISALIGNED_SLOW (2 << 0) #define RISCV_HWPROBE_MISALIGNED_FAST (3 << 0) #define RISCV_HWPROBE_MISALIGNED_UNSUPPORTED (4 << 0) #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
int __riscv_hwprobe (struct riscv_hwprobe *pairs, long pair_count, long cpu_count, unsigned long *cpus, unsigned long flags) {
return syscall(__NR_riscv_hwprobe, pairs, pair_count, cpu_count, cpus, flags); }
int main(void) { struct riscv_hwprobe pairs[3];
pairs[0].key = RISCV_HWPROBE_KEY_MVENDORID; pairs[1].key = RISCV_HWPROBE_KEY_MARCHID; pairs[2].key = RISCV_HWPROBE_KEY_MIMPID; if (__riscv_hwprobe(pairs, 3, 0, NULL, 0) != 0) { printf("syscall failed"); return -1; }
printf("vendorid 0x%x, archid 0x%x, impid 0x%x\n", pairs[0].value, pairs[1].value, pairs[2].value);
pairs[0].key = RISCV_HWPROBE_KEY_CPUPERF_0; pairs[1].key = RISCV_HWPROBE_KEY_BASE_BEHAVIOR; pairs[2].key = RISCV_HWPROBE_KEY_IMA_EXT_0; if (__riscv_hwprobe(&pairs[0], 3, 0, NULL, 0) != 0) { printf("syscall failed"); return -1; }
printf("ima-behavior %d, f+d %d, c %d, misaligned access: %s\n", ((pairs[1].value & RISCV_HWPROBE_BASE_BEHAVIOR_IMA) == RISCV_HWPROBE_BASE_BEHAVIOR_IMA), ((pairs[2].value & RISCV_HWPROBE_IMA_FD) == RISCV_HWPROBE_IMA_FD), ((pairs[2].value & RISCV_HWPROBE_IMA_C) == RISCV_HWPROBE_IMA_C), ((pairs[0].value & RISCV_HWPROBE_MISALIGNED_FAST) == RISCV_HWPROBE_MISALIGNED_FAST) ? "fast" : "not-fast" );
return 0; }