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physmem: fd-based shared memory

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Create MAP_SHARED RAMBlocks by mmap'ing a file descriptor rather than using
MAP_ANON, so the memory can be accessed in another process by passing and
mmap'ing the fd.  This will allow CPR to support memory-backend-ram and
memory-backend-shm objects, provided the user creates them with share=on.

Use memfd_create if available because it has no constraints.  If not, use
POSIX shm_open.  However, allocation on the opened fd may fail if the shm
mount size is too small, even if the system has free memory, so for backwards
compatibility fall back to qemu_anon_ram_alloc/MAP_ANON on failure.

For backwards compatibility on Windows, always use MAP_ANON.  share=on has
no purpose there, but the syntax is accepted, and must continue to work.

Lastly, quietly fall back to MAP_ANON if the system does not support
qemu_ram_alloc_from_fd.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-5-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>
This commit is contained in:
Steve Sistare 2025-01-15 11:00:30 -08:00 • committed by Fabiano Rosas
parent 3ec0214816
commit 9fb40bb962
3 changed files with 70 additions and 4 deletions
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system/trace-events
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@ -33,6 +33,7 @@ address_space_map(void *as, uint64_t addr, uint64_t len, bool is_write, uint32_t
find_ram_offset(uint64_t size, uint64_t offset) "size: 0x%" PRIx64 " @ 0x%" PRIx64
find_ram_offset_loop(uint64_t size, uint64_t candidate, uint64_t offset, uint64_t next, uint64_t mingap) "trying size: 0x%" PRIx64 " @ 0x%" PRIx64 ", offset: 0x%" PRIx64" next: 0x%" PRIx64 " mingap: 0x%" PRIx64
ram_block_discard_range(const char *rbname, void *hva, size_t length, bool need_madvise, bool need_fallocate, int ret) "%s@%p + 0x%zx: madvise: %d fallocate: %d ret: %d"
qemu_ram_alloc_shared(const char *name, size_t size, size_t max_size, int fd, void *host) "%s size %zu max_size %zu fd %d host %p"
# cpus.c
vm_stop_flush_all(int ret) "ret %d"