Yo,
I mentioned this last night to Heinrich on IRC, supports_extension() is broken for ISA strings longer than 32 characters. M-Mode U-Boot doesn't parse a devicetree, so this doesn't apply there, but for S-mode supports_extension() looks like:
static inline bool supports_extension(char ext) {
struct udevice *dev; char desc[32]; int i;
uclass_find_first_device(UCLASS_CPU, &dev); if (!dev) { debug("unable to find the RISC-V cpu device\n"); return false; } if (!cpu_get_desc(dev, desc, sizeof(desc))) { /* * skip the first 4 characters (rv32|rv64) and * check until underscore */ for (i = 4; i < sizeof(desc); i++) { if (desc[i] == '_' || desc[i] == '\0') break; if (desc[i] == ext) return true; } }
return false; }
cpu_get_desc is implemented by riscv_cpu_get_desc(): static int riscv_cpu_get_desc(const struct udevice *dev, char *buf, int size) { const char *isa;
isa = dev_read_string(dev, "riscv,isa"); if (size < (strlen(isa) + 1)) return -ENOSPC;
strcpy(buf, isa);
return 0; }
On most extant systems, riscv,isa is a pretty short string - between 10 and 20 characters. In QEMU's default virt machine however, we get: riscv,isa = "rv64imafdch_zicbom_zicbop_zicboz_zicntr_zicsr_zifencei_zihintntl_zihintpause_zihpm_zawrs_zfa_zca_zcd_zba_zbb_zbc_zbs_sstc_svadu";
Since desc can only contain 32 elements, the size < strlen() test fails and cpu_get_desc() returns an error and supports_extension() in turn returns false.
Currently, in S-Mode, there's only two extensions that U-Boot ever looks for and they lie inside the single letter section, so 32 charcters would be sufficiently sized, if cpu_get_desc() supported undersized buffers.
I came across this while adding support for a different way of detecting ISA extensions, rather than running into an actual problem because U-Boot seems not to actually make use of supports_extension() other than enabling an FPU that there seem to be no users of in U-Boot at present. I also assume that using U-Boot in QEMU is somewhat of a rare case, given with virt you can boot an OS kernel directly. That'd make the impact of this problem pretty low, given I just happened to notice that in my test environment no extensions were being detected and the operation of U-Boot seemed unaffected.
I'm mostly just wondering if, given the impact seems to be rather low, if I should "bother" making a minimal fix for this that would be applied to master (or backported? not 100% sure of the release process for U-Boot), or if I can just fix it in passing while making "riscv,isa" optional?
A minimal fix would look something like the following:
diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c index 8445c5823e..df508ac4a1 100644 --- a/arch/riscv/cpu/cpu.c +++ b/arch/riscv/cpu/cpu.c @@ -39,7 +39,6 @@ static inline bool supports_extension(char ext) return csr_read(CSR_MISA) & (1 << (ext - 'a')); #elif CONFIG_CPU struct udevice *dev; - char desc[32]; int i;
uclass_find_first_device(UCLASS_CPU, &dev); @@ -47,15 +46,16 @@ static inline bool supports_extension(char ext) debug("unable to find the RISC-V cpu device\n"); return false; } - if (!cpu_get_desc(dev, desc, sizeof(desc))) { + const char *isa = dev_read_string(dev, "riscv,isa"); + if (isa) { /* * skip the first 4 characters (rv32|rv64) and * check until underscore */ - for (i = 4; i < sizeof(desc); i++) { - if (desc[i] == '_' || desc[i] == '\0') + for (i = 4; i < strlen(isa); i++) { + if (isa[i] == '_' || isa[i] == '\0') break; - if (desc[i] == ext) + if (isa[i] == ext) return true; } }
Cheers, Conor.