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RISC-V: detect virtual address space at runtime using hwprobe #1296

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RISC-V: detect virtual address space at runtime using hwprobe #1296

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10 changes: 0 additions & 10 deletions CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -422,16 +422,6 @@ else()
# list(APPEND mi_defines MI_WIN_INIT_USE_CRT_TLS=1)
endif()

# Check /proc/cpuinfo for an SV39 MMU and limit the virtual address bits.
# (this will skip the aligned hinting in that case. Issue #939, #949)
if (EXISTS /proc/cpuinfo)
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file(STRINGS /proc/cpuinfo mi_sv39_mmu REGEX "^mmu[ \t]+:[ \t]+sv39$")
if (mi_sv39_mmu)
MESSAGE( STATUS "Set virtual address bits to 39 (SV39 MMU detected)" )
list(APPEND mi_defines MI_DEFAULT_VIRTUAL_ADDRESS_BITS=39)
endif()
endif()

# On Haiku use `-DCMAKE_INSTALL_PREFIX` instead, issue #788
# if(CMAKE_SYSTEM_NAME MATCHES "Haiku")
# SET(CMAKE_INSTALL_LIBDIR ~/config/non-packaged/lib)
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54 changes: 54 additions & 0 deletions src/prim/unix/prim.c
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Expand Up @@ -41,6 +41,13 @@ terms of the MIT license. A copy of the license can be found in the file
#else
#include <sys/mman.h>
#endif
#if defined(__riscv) || defined(_M_RISCV)

@daanx daanx Jun 23, 2026

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We cannot use extensions like __has_include generally as it requires C23 (and mimalloc assumes C11). I think we should use the cmake CHECK_INCLUDE_FILES and pass an extra define MI_HAS_ASM_HWPROBE_H such we can test in the sources for it.

(mmm, but maybe riscV is so recent that we can assume that all C compilers on riscV support __has_include ? ... maybe we can make an exception in that case as __has_include is quite clean?)

@aurel32 aurel32 Jun 24, 2026

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I have seen it used in an other place so I assumed it was fine, but I realized now that it was guarded by defined(__APPLE__). I'll fix that as you suggested

#if __has_include(<sys/hwprobe.h>)
#include <sys/hwprobe.h>
#elif __has_include(<asm/hwprobe.h>)
#include <asm/hwprobe.h>
#endif
#endif
#elif defined(__APPLE__)
#include <AvailabilityMacros.h>
#include <TargetConditionals.h>
Expand Down Expand Up @@ -187,6 +194,52 @@ static void unix_detect_physical_memory( size_t page_size, size_t* physical_memo
#endif
}

// Detect the virtual address bits (currently Linux/RISC-V only)
static size_t unix_detect_virtual_address_bits(void)
{
#if defined(__riscv) || defined(_M_RISCV)
#ifdef RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS
struct riscv_hwprobe probe = {
.key = RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS,
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};

// Prefer the GNU libc interface if available, as it can also use the VDSO
#if __has_include(<sys/hwprobe.h>)
if (__riscv_hwprobe(&probe, 1, 0, NULL, 0) == 0) {
#else
if (syscall(__NR_riscv_hwprobe, &probe, 1, 0, NULL, 0) == 0) {
#endif
// If a key is unknown to the kernel, its key field will be cleared to -1.
if (probe.key != -1) {
return MI_SIZE_BITS - mi_clz((uintptr_t)probe.value);
}
}
#endif

// Fallback to checking /proc/cpuinfo for older kernels
int fd = mi_prim_open("/proc/cpuinfo", O_RDONLY);

@daanx daanx Jun 23, 2026

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use const int fd

@aurel32 aurel32 Jun 24, 2026

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ok, i'll fix that

if (fd >= 0) {
char buf[4096];

@daanx daanx Jun 23, 2026

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That is a large buffer on the stack; maybe 1024 bytes is enough to find if svXX is there?

@aurel32 aurel32 Jun 24, 2026

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That works fine for now on a recent CPU, but that might be tight if more extensions are added. Here is for instance the first 1024 bytes on a SpacemiT K3:

processor       : 0
hart            : 0
isa             : rv64imafdcvh_zicbom_zicbop_zicboz_ziccrse_zicntr_zicond_zicsr_zifencei_zihintntl_zihintpause_zihpm_zimop_zaamo_zalrsc_zawrs_zfa_zfbfmin_zfh_zfhmin_zca_zcb_zcd_zcmop_zba_zbb_zbc_zbs_zkt_zvbb_zvbc_zve32f_zve32x_zve64d_zve64f_zve64x_zvfbfmin_zvfbfwma_zvfh_zvfhmin_zvkb_zvkg_zvkned_zvknha_zvknhb_zvksed_zvksh_zvkt_smaia_smstateen_ssaia_sscofpmf_ssnpm_sstc_svade_svinval_svnapot_svpbmt
mmu             : sv39
uarch           : spacemit,x100
mvendorid       : 0x710
marchid         : 0x8000000058000002
mimpid          : 0x33d8a600
hart isa        : rv64imafdcvh_zicbom_zicbop_zicboz_ziccrse_zicntr_zicond_zicsr_zifencei_zihintntl_zihintpause_zihpm_zimop_zaamo_zalrsc_zawrs_zfa_zfbfmin_zfh_zfhmin_zca_zcb_zcd_zcmop_zba_zbb_zbc_zbs_zkt_zvbb_zvbc_zve32f_zve32x_zve64d_zve64f_zve64x_zvfbfmin_zvfbfwma_zvfh_zvfhmin_zvkb_zvkg_zvkned_zvknha_zvknhb_zvksed_zvksh_zvkt_smaia_smstateen_ssaia_sscofpmf_ssnpm_sstc_svade_svinval_svnapot_svpbmt

processor       : 1
hart            : 1

Would that be acceptable to go down to 2048 bytes instead?

ssize_t nread = mi_prim_read(fd, &buf, sizeof(buf));
mi_prim_close(fd);
if (nread >= 1) {
if (_mi_strnstr(buf, nread, "sv39")) {

@daanx daanx Jun 23, 2026

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Maybe we should also test if nread <= sizeof(buf) just in case.

@aurel32 aurel32 Jun 24, 2026

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Thanks, i'll fix that

return 39;
}
if (_mi_strnstr(buf, nread, "sv48")) {
return 48;
}
if (_mi_strnstr(buf, nread, "sv57")) {
return 57;
}
}
}
#endif

// default: MI_MAX_VABITS
return MI_MAX_VABITS;
}

void _mi_prim_mem_init( mi_os_mem_config_t* config )
{
long psize = sysconf(_SC_PAGESIZE);
Expand All @@ -199,6 +252,7 @@ void _mi_prim_mem_init( mi_os_mem_config_t* config )
config->has_overcommit = unix_detect_overcommit();
config->has_partial_free = true; // mmap can free in parts
config->has_virtual_reserve = true; // todo: check if this true for NetBSD? (for anonymous mmap with PROT_NONE)
config->virtual_address_bits = unix_detect_virtual_address_bits();

// disable transparent huge pages for this process?
#if (defined(__linux__) || defined(__ANDROID__)) && defined(PR_GET_THP_DISABLE)
Expand Down