/* * mmap support for qemu * * Copyright (c) 2003 Fabrice Bellard * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, see . */ #include "qemu/osdep.h" #include "trace.h" #include "exec/log.h" #include "qemu.h" #include "user-internals.h" #include "user-mmap.h" #include "target_mman.h" static pthread_mutex_t mmap_mutex = PTHREAD_MUTEX_INITIALIZER; static __thread int mmap_lock_count; void mmap_lock(void) { if (mmap_lock_count++ == 0) { pthread_mutex_lock(&mmap_mutex); } } void mmap_unlock(void) { if (--mmap_lock_count == 0) { pthread_mutex_unlock(&mmap_mutex); } } bool have_mmap_lock(void) { return mmap_lock_count > 0 ? true : false; } /* Grab lock to make sure things are in a consistent state after fork(). */ void mmap_fork_start(void) { if (mmap_lock_count) abort(); pthread_mutex_lock(&mmap_mutex); } void mmap_fork_end(int child) { if (child) pthread_mutex_init(&mmap_mutex, NULL); else pthread_mutex_unlock(&mmap_mutex); } /* * Validate target prot bitmask. * Return the prot bitmask for the host in *HOST_PROT. * Return 0 if the target prot bitmask is invalid, otherwise * the internal qemu page_flags (which will include PAGE_VALID). */ static int validate_prot_to_pageflags(int *host_prot, int prot) { int valid = PROT_READ | PROT_WRITE | PROT_EXEC | TARGET_PROT_SEM; int page_flags = (prot & PAGE_BITS) | PAGE_VALID; /* * For the host, we need not pass anything except read/write/exec. * While PROT_SEM is allowed by all hosts, it is also ignored, so * don't bother transforming guest bit to host bit. Any other * target-specific prot bits will not be understood by the host * and will need to be encoded into page_flags for qemu emulation. * * Pages that are executable by the guest will never be executed * by the host, but the host will need to be able to read them. */ *host_prot = (prot & (PROT_READ | PROT_WRITE)) | (prot & PROT_EXEC ? PROT_READ : 0); #ifdef TARGET_AARCH64 { ARMCPU *cpu = ARM_CPU(thread_cpu); /* * The PROT_BTI bit is only accepted if the cpu supports the feature. * Since this is the unusual case, don't bother checking unless * the bit has been requested. If set and valid, record the bit * within QEMU's page_flags. */ if ((prot & TARGET_PROT_BTI) && cpu_isar_feature(aa64_bti, cpu)) { valid |= TARGET_PROT_BTI; page_flags |= PAGE_BTI; } /* Similarly for the PROT_MTE bit. */ if ((prot & TARGET_PROT_MTE) && cpu_isar_feature(aa64_mte, cpu)) { valid |= TARGET_PROT_MTE; page_flags |= PAGE_MTE; } } #elif defined(TARGET_HPPA) valid |= PROT_GROWSDOWN | PROT_GROWSUP; #endif return prot & ~valid ? 0 : page_flags; } /* NOTE: all the constants are the HOST ones, but addresses are target. */ int target_mprotect(abi_ulong start, abi_ulong len, int target_prot) { abi_ulong end, host_start, host_end, addr; int prot1, ret, page_flags, host_prot; trace_target_mprotect(start, len, target_prot); if ((start & ~TARGET_PAGE_MASK) != 0) { return -TARGET_EINVAL; } page_flags = validate_prot_to_pageflags(&host_prot, target_prot); if (!page_flags) { return -TARGET_EINVAL; } len = TARGET_PAGE_ALIGN(len); end = start + len; if (!guest_range_valid_untagged(start, len)) { return -TARGET_ENOMEM; } if (len == 0) { return 0; } mmap_lock(); host_start = start & qemu_host_page_mask; host_end = HOST_PAGE_ALIGN(end); if (start > host_start) { /* handle host page containing start */ prot1 = host_prot; for (addr = host_start; addr < start; addr += TARGET_PAGE_SIZE) { prot1 |= page_get_flags(addr); } if (host_end == host_start + qemu_host_page_size) { for (addr = end; addr < host_end; addr += TARGET_PAGE_SIZE) { prot1 |= page_get_flags(addr); } end = host_end; } ret = mprotect(g2h_untagged(host_start), qemu_host_page_size, prot1 & PAGE_BITS); if (ret != 0) { goto error; } host_start += qemu_host_page_size; } if (end < host_end) { prot1 = host_prot; for (addr = end; addr < host_end; addr += TARGET_PAGE_SIZE) { prot1 |= page_get_flags(addr); } ret = mprotect(g2h_untagged(host_end - qemu_host_page_size), qemu_host_page_size, prot1 & PAGE_BITS); if (ret != 0) { goto error; } host_end -= qemu_host_page_size; } /* handle the pages in the middle */ if (host_start < host_end) { ret = mprotect(g2h_untagged(host_start), host_end - host_start, host_prot); if (ret != 0) { goto error; } } page_set_flags(start, start + len - 1, page_flags); ret = 0; error: mmap_unlock(); return ret; } /* map an incomplete host page */ static int mmap_frag(abi_ulong real_start, abi_ulong start, abi_ulong end, int prot, int flags, int fd, abi_ulong offset) { abi_ulong real_end, addr; void *host_start; int prot1, prot_new; real_end = real_start + qemu_host_page_size; host_start = g2h_untagged(real_start); /* get the protection of the target pages outside the mapping */ prot1 = 0; for(addr = real_start; addr < real_end; addr++) { if (addr < start || addr >= end) prot1 |= page_get_flags(addr); } if (prot1 == 0) { /* no page was there, so we allocate one */ void *p = mmap(host_start, qemu_host_page_size, prot, flags | MAP_ANONYMOUS, -1, 0); if (p == MAP_FAILED) return -1; prot1 = prot; } prot1 &= PAGE_BITS; prot_new = prot | prot1; if (!(flags & MAP_ANONYMOUS)) { /* msync() won't work here, so we return an error if write is possible while it is a shared mapping */ if ((flags & MAP_TYPE) == MAP_SHARED && (prot & PROT_WRITE)) return -1; /* adjust protection to be able to read */ if (!(prot1 & PROT_WRITE)) mprotect(host_start, qemu_host_page_size, prot1 | PROT_WRITE); /* read the corresponding file data */ if (pread(fd, g2h_untagged(start), end - start, offset) == -1) return -1; /* put final protection */ if (prot_new != (prot1 | PROT_WRITE)) mprotect(host_start, qemu_host_page_size, prot_new); } else { if (prot_new != prot1) { mprotect(host_start, qemu_host_page_size, prot_new); } if (prot_new & PROT_WRITE) { memset(g2h_untagged(start), 0, end - start); } } return 0; } #if HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 64 #ifdef TARGET_AARCH64 # define TASK_UNMAPPED_BASE 0x5500000000 #else # define TASK_UNMAPPED_BASE (1ul << 38) #endif #else #ifdef TARGET_HPPA # define TASK_UNMAPPED_BASE 0xfa000000 #else # define TASK_UNMAPPED_BASE 0x40000000 #endif #endif abi_ulong mmap_next_start = TASK_UNMAPPED_BASE; unsigned long last_brk; /* Subroutine of mmap_find_vma, used when we have pre-allocated a chunk of guest address space. */ static abi_ulong mmap_find_vma_reserved(abi_ulong start, abi_ulong size, abi_ulong align) { abi_ulong addr, end_addr, incr = qemu_host_page_size; int prot; bool looped = false; if (size > reserved_va) { return (abi_ulong)-1; } /* Note that start and size have already been aligned by mmap_find_vma. */ end_addr = start + size; if (start > reserved_va - size) { /* Start at the top of the address space. */ end_addr = ((reserved_va - size) & -align) + size; looped = true; } /* Search downward from END_ADDR, checking to see if a page is in use. */ addr = end_addr; while (1) { addr -= incr; if (addr > end_addr) { if (looped) { /* Failure. The entire address space has been searched. */ return (abi_ulong)-1; } /* Re-start at the top of the address space. */ addr = end_addr = ((reserved_va - size) & -align) + size; looped = true; } else { prot = page_get_flags(addr); if (prot) { /* Page in use. Restart below this page. */ addr = end_addr = ((addr - size) & -align) + size; } else if (addr && addr + size == end_addr) { /* Success! All pages between ADDR and END_ADDR are free. */ if (start == mmap_next_start) { mmap_next_start = addr; } return addr; } } } } /* * Find and reserve a free memory area of size 'size'. The search * starts at 'start'. * It must be called with mmap_lock() held. * Return -1 if error. */ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size, abi_ulong align) { void *ptr, *prev; abi_ulong addr; int wrapped, repeat; align = MAX(align, qemu_host_page_size); /* If 'start' == 0, then a default start address is used. */ if (start == 0) { start = mmap_next_start; } else { start &= qemu_host_page_mask; } start = ROUND_UP(start, align); size = HOST_PAGE_ALIGN(size); if (reserved_va) { return mmap_find_vma_reserved(start, size, align); } addr = start; wrapped = repeat = 0; prev = 0; for (;; prev = ptr) { /* * Reserve needed memory area to avoid a race. * It should be discarded using: * - mmap() with MAP_FIXED flag * - mremap() with MREMAP_FIXED flag * - shmat() with SHM_REMAP flag */ ptr = mmap(g2h_untagged(addr), size, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE, -1, 0); /* ENOMEM, if host address space has no memory */ if (ptr == MAP_FAILED) { return (abi_ulong)-1; } /* Count the number of sequential returns of the same address. This is used to modify the search algorithm below. */ repeat = (ptr == prev ? repeat + 1 : 0); if (h2g_valid(ptr + size - 1)) { addr = h2g(ptr); if ((addr & (align - 1)) == 0) { /* Success. */ if (start == mmap_next_start && addr >= TASK_UNMAPPED_BASE) { mmap_next_start = addr + size; } return addr; } /* The address is not properly aligned for the target. */ switch (repeat) { case 0: /* Assume the result that the kernel gave us is the first with enough free space, so start again at the next higher target page. */ addr = ROUND_UP(addr, align); break; case 1: /* Sometimes the kernel decides to perform the allocation at the top end of memory instead. */ addr &= -align; break; case 2: /* Start over at low memory. */ addr = 0; break; default: /* Fail. This unaligned block must the last. */ addr = -1; break; } } else { /* Since the result the kernel gave didn't fit, start again at low memory. If any repetition, fail. */ addr = (repeat ? -1 : 0); } /* Unmap and try again. */ munmap(ptr, size); /* ENOMEM if we checked the whole of the target address space. */ if (addr == (abi_ulong)-1) { return (abi_ulong)-1; } else if (addr == 0) { if (wrapped) { return (abi_ulong)-1; } wrapped = 1; /* Don't actually use 0 when wrapping, instead indicate that we'd truly like an allocation in low memory. */ addr = (mmap_min_addr > TARGET_PAGE_SIZE ? TARGET_PAGE_ALIGN(mmap_min_addr) : TARGET_PAGE_SIZE); } else if (wrapped && addr >= start) { return (abi_ulong)-1; } } } /* NOTE: all the constants are the HOST ones */ abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot, int flags, int fd, abi_ulong offset) { abi_ulong ret, end, real_start, real_end, retaddr, host_offset, host_len, passthrough_start = -1, passthrough_end = -1; int page_flags, host_prot; mmap_lock(); trace_target_mmap(start, len, target_prot, flags, fd, offset); if (!len) { errno = EINVAL; goto fail; } page_flags = validate_prot_to_pageflags(&host_prot, target_prot); if (!page_flags) { errno = EINVAL; goto fail; } /* Also check for overflows... */ len = TARGET_PAGE_ALIGN(len); if (!len) { errno = ENOMEM; goto fail; } if (offset & ~TARGET_PAGE_MASK) { errno = EINVAL; goto fail; } /* * If we're mapping shared memory, ensure we generate code for parallel * execution and flush old translations. This will work up to the level * supported by the host -- anything that requires EXCP_ATOMIC will not * be atomic with respect to an external process. */ if (flags & MAP_SHARED) { CPUState *cpu = thread_cpu; if (!(cpu->tcg_cflags & CF_PARALLEL)) { cpu->tcg_cflags |= CF_PARALLEL; tb_flush(cpu); } } real_start = start & qemu_host_page_mask; host_offset = offset & qemu_host_page_mask; /* If the user is asking for the kernel to find a location, do that before we truncate the length for mapping files below. */ if (!(flags & MAP_FIXED)) { host_len = len + offset - host_offset; host_len = HOST_PAGE_ALIGN(host_len); start = mmap_find_vma(real_start, host_len, TARGET_PAGE_SIZE); if (start == (abi_ulong)-1) { errno = ENOMEM; goto fail; } } /* When mapping files into a memory area larger than the file, accesses to pages beyond the file size will cause a SIGBUS. For example, if mmaping a file of 100 bytes on a host with 4K pages emulating a target with 8K pages, the target expects to be able to access the first 8K. But the host will trap us on any access beyond 4K. When emulating a target with a larger page-size than the hosts, we may need to truncate file maps at EOF and add extra anonymous pages up to the targets page boundary. */ if ((qemu_real_host_page_size() < qemu_host_page_size) && !(flags & MAP_ANONYMOUS)) { struct stat sb; if (fstat (fd, &sb) == -1) goto fail; /* Are we trying to create a map beyond EOF?. */ if (offset + len > sb.st_size) { /* If so, truncate the file map at eof aligned with the hosts real pagesize. Additional anonymous maps will be created beyond EOF. */ len = REAL_HOST_PAGE_ALIGN(sb.st_size - offset); } } if (!(flags & MAP_FIXED)) { unsigned long host_start; void *p; host_len = len + offset - host_offset; host_len = HOST_PAGE_ALIGN(host_len); /* Note: we prefer to control the mapping address. It is especially important if qemu_host_page_size > qemu_real_host_page_size */ p = mmap(g2h_untagged(start), host_len, host_prot, flags | MAP_FIXED | MAP_ANONYMOUS, -1, 0); if (p == MAP_FAILED) { goto fail; } /* update start so that it points to the file position at 'offset' */ host_start = (unsigned long)p; if (!(flags & MAP_ANONYMOUS)) { p = mmap(g2h_untagged(start), len, host_prot, flags | MAP_FIXED, fd, host_offset); if (p == MAP_FAILED) { munmap(g2h_untagged(start), host_len); goto fail; } host_start += offset - host_offset; } start = h2g(host_start); passthrough_start = start; passthrough_end = start + len; } else { if (start & ~TARGET_PAGE_MASK) { errno = EINVAL; goto fail; } end = start + len; real_end = HOST_PAGE_ALIGN(end); /* * Test if requested memory area fits target address space * It can fail only on 64-bit host with 32-bit target. * On any other target/host host mmap() handles this error correctly. */ if (end < start || !guest_range_valid_untagged(start, len)) { errno = ENOMEM; goto fail; } /* worst case: we cannot map the file because the offset is not aligned, so we read it */ if (!(flags & MAP_ANONYMOUS) && (offset & ~qemu_host_page_mask) != (start & ~qemu_host_page_mask)) { /* msync() won't work here, so we return an error if write is possible while it is a shared mapping */ if ((flags & MAP_TYPE) == MAP_SHARED && (host_prot & PROT_WRITE)) { errno = EINVAL; goto fail; } retaddr = target_mmap(start, len, target_prot | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (retaddr == -1) goto fail; if (pread(fd, g2h_untagged(start), len, offset) == -1) goto fail; if (!(host_prot & PROT_WRITE)) { ret = target_mprotect(start, len, target_prot); assert(ret == 0); } goto the_end; } /* handle the start of the mapping */ if (start > real_start) { if (real_end == real_start + qemu_host_page_size) { /* one single host page */ ret = mmap_frag(real_start, start, end, host_prot, flags, fd, offset); if (ret == -1) goto fail; goto the_end1; } ret = mmap_frag(real_start, start, real_start + qemu_host_page_size, host_prot, flags, fd, offset); if (ret == -1) goto fail; real_start += qemu_host_page_size; } /* handle the end of the mapping */ if (end < real_end) { ret = mmap_frag(real_end - qemu_host_page_size, real_end - qemu_host_page_size, end, host_prot, flags, fd, offset + real_end - qemu_host_page_size - start); if (ret == -1) goto fail; real_end -= qemu_host_page_size; } /* map the middle (easier) */ if (real_start < real_end) { void *p; unsigned long offset1; if (flags & MAP_ANONYMOUS) offset1 = 0; else offset1 = offset + real_start - start; p = mmap(g2h_untagged(real_start), real_end - real_start, host_prot, flags, fd, offset1); if (p == MAP_FAILED) goto fail; passthrough_start = real_start; passthrough_end = real_end; } } the_end1: if (flags & MAP_ANONYMOUS) { page_flags |= PAGE_ANON; } page_flags |= PAGE_RESET; if (passthrough_start == passthrough_end) { page_set_flags(start, start + len - 1, page_flags); } else { if (start < passthrough_start) { page_set_flags(start, passthrough_start - 1, page_flags); } page_set_flags(passthrough_start, passthrough_end - 1, page_flags | PAGE_PASSTHROUGH); if (passthrough_end < start + len) { page_set_flags(passthrough_end, start + len - 1, page_flags); } } the_end: trace_target_mmap_complete(start); if (qemu_loglevel_mask(CPU_LOG_PAGE)) { FILE *f = qemu_log_trylock(); if (f) { fprintf(f, "page layout changed following mmap\n"); page_dump(f); qemu_log_unlock(f); } } mmap_unlock(); return start; fail: mmap_unlock(); return -1; } static void mmap_reserve(abi_ulong start, abi_ulong size) { abi_ulong real_start; abi_ulong real_end; abi_ulong addr; abi_ulong end; int prot; real_start = start & qemu_host_page_mask; real_end = HOST_PAGE_ALIGN(start + size); end = start + size; if (start > real_start) { /* handle host page containing start */ prot = 0; for (addr = real_start; addr < start; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } if (real_end == real_start + qemu_host_page_size) { for (addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } end = real_end; } if (prot != 0) real_start += qemu_host_page_size; } if (end < real_end) { prot = 0; for (addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } if (prot != 0) real_end -= qemu_host_page_size; } if (real_start != real_end) { mmap(g2h_untagged(real_start), real_end - real_start, PROT_NONE, MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0); } } int target_munmap(abi_ulong start, abi_ulong len) { abi_ulong end, real_start, real_end, addr; int prot, ret; trace_target_munmap(start, len); if (start & ~TARGET_PAGE_MASK) return -TARGET_EINVAL; len = TARGET_PAGE_ALIGN(len); if (len == 0 || !guest_range_valid_untagged(start, len)) { return -TARGET_EINVAL; } mmap_lock(); end = start + len; real_start = start & qemu_host_page_mask; real_end = HOST_PAGE_ALIGN(end); if (start > real_start) { /* handle host page containing start */ prot = 0; for(addr = real_start; addr < start; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } if (real_end == real_start + qemu_host_page_size) { for(addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } end = real_end; } if (prot != 0) real_start += qemu_host_page_size; } if (end < real_end) { prot = 0; for(addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) { prot |= page_get_flags(addr); } if (prot != 0) real_end -= qemu_host_page_size; } ret = 0; /* unmap what we can */ if (real_start < real_end) { if (reserved_va) { mmap_reserve(real_start, real_end - real_start); } else { ret = munmap(g2h_untagged(real_start), real_end - real_start); } } if (ret == 0) { page_set_flags(start, start + len - 1, 0); } mmap_unlock(); return ret; } abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, abi_ulong new_size, unsigned long flags, abi_ulong new_addr) { int prot; void *host_addr; if (!guest_range_valid_untagged(old_addr, old_size) || ((flags & MREMAP_FIXED) && !guest_range_valid_untagged(new_addr, new_size)) || ((flags & MREMAP_MAYMOVE) == 0 && !guest_range_valid_untagged(old_addr, new_size))) { errno = ENOMEM; return -1; } mmap_lock(); if (flags & MREMAP_FIXED) { host_addr = mremap(g2h_untagged(old_addr), old_size, new_size, flags, g2h_untagged(new_addr)); if (reserved_va && host_addr != MAP_FAILED) { /* If new and old addresses overlap then the above mremap will already have failed with EINVAL. */ mmap_reserve(old_addr, old_size); } } else if (flags & MREMAP_MAYMOVE) { abi_ulong mmap_start; mmap_start = mmap_find_vma(0, new_size, TARGET_PAGE_SIZE); if (mmap_start == -1) { errno = ENOMEM; host_addr = MAP_FAILED; } else { host_addr = mremap(g2h_untagged(old_addr), old_size, new_size, flags | MREMAP_FIXED, g2h_untagged(mmap_start)); if (reserved_va) { mmap_reserve(old_addr, old_size); } } } else { int prot = 0; if (reserved_va && old_size < new_size) { abi_ulong addr; for (addr = old_addr + old_size; addr < old_addr + new_size; addr++) { prot |= page_get_flags(addr); } } if (prot == 0) { host_addr = mremap(g2h_untagged(old_addr), old_size, new_size, flags); if (host_addr != MAP_FAILED) { /* Check if address fits target address space */ if (!guest_range_valid_untagged(h2g(host_addr), new_size)) { /* Revert mremap() changes */ host_addr = mremap(g2h_untagged(old_addr), new_size, old_size, flags); errno = ENOMEM; host_addr = MAP_FAILED; } else if (reserved_va && old_size > new_size) { mmap_reserve(old_addr + old_size, old_size - new_size); } } } else { errno = ENOMEM; host_addr = MAP_FAILED; } } if (host_addr == MAP_FAILED) { new_addr = -1; } else { new_addr = h2g(host_addr); prot = page_get_flags(old_addr); page_set_flags(old_addr, old_addr + old_size - 1, 0); page_set_flags(new_addr, new_addr + new_size - 1, prot | PAGE_VALID | PAGE_RESET); } mmap_unlock(); return new_addr; } static bool can_passthrough_madvise(abi_ulong start, abi_ulong end) { ulong addr; if ((start | end) & ~qemu_host_page_mask) { return false; } for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) { if (!(page_get_flags(addr) & PAGE_PASSTHROUGH)) { return false; } } return true; } abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice) { abi_ulong len, end; int ret = 0; if (start & ~TARGET_PAGE_MASK) { return -TARGET_EINVAL; } len = TARGET_PAGE_ALIGN(len_in); if (len_in && !len) { return -TARGET_EINVAL; } end = start + len; if (end < start) { return -TARGET_EINVAL; } if (end == start) { return 0; } if (!guest_range_valid_untagged(start, len)) { return -TARGET_EINVAL; } /* Translate for some architectures which have different MADV_xxx values */ switch (advice) { case TARGET_MADV_DONTNEED: /* alpha */ advice = MADV_DONTNEED; break; case TARGET_MADV_WIPEONFORK: /* parisc */ advice = MADV_WIPEONFORK; break; case TARGET_MADV_KEEPONFORK: /* parisc */ advice = MADV_KEEPONFORK; break; /* we do not care about the other MADV_xxx values yet */ } /* * Most advice values are hints, so ignoring and returning success is ok. * * However, some advice values such as MADV_DONTNEED, MADV_WIPEONFORK and * MADV_KEEPONFORK are not hints and need to be emulated. * * A straight passthrough for those may not be safe because qemu sometimes * turns private file-backed mappings into anonymous mappings. * can_passthrough_madvise() helps to check if a passthrough is possible by * comparing mappings that are known to have the same semantics in the host * and the guest. In this case passthrough is safe. * * We pass through MADV_WIPEONFORK and MADV_KEEPONFORK if possible and * return failure if not. * * MADV_DONTNEED is passed through as well, if possible. * If passthrough isn't possible, we nevertheless (wrongly!) return * success, which is broken but some userspace programs fail to work * otherwise. Completely implementing such emulation is quite complicated * though. */ mmap_lock(); switch (advice) { case MADV_WIPEONFORK: case MADV_KEEPONFORK: ret = -EINVAL; /* fall through */ case MADV_DONTNEED: if (can_passthrough_madvise(start, end)) { ret = get_errno(madvise(g2h_untagged(start), len, advice)); if ((advice == MADV_DONTNEED) && (ret == 0)) { page_reset_target_data(start, start + len - 1); } } } mmap_unlock(); return ret; }