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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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@res2k res2k May 18, 2026

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Perhaps turn that into an explicit build time option? That way, people targeting an SV39 can at least explicitly request an appropriate number of bits.
(TBF, I have no idea how large that group may be.)

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@res2k res2k May 18, 2026

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... targeting an SV39 and an older kernel.

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@aurel32 aurel32 May 19, 2026

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I am not a big fan of configure options like that, because people don't really look at it until things are broken. I prefer when things work out of the box, but I understand your concern about supporting older kernels. What about adding a fallback parsing /proc/cpuinfo at runtime for older kernels?

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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33 changes: 33 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)
#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 +194,31 @@ 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)
{
#ifdef RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS
struct riscv_hwprobe probe = {
.key = RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS,
};

// 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

// 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 +231,7 @@ 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)
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