Hi, Did some digging, correction(s) below. On Thu, Nov 24, 2022 at 05:14:33PM +0000, Alexandru Elisei wrote: > Hi, > > On Thu, Nov 24, 2022 at 03:19:34PM +0000, Fuad Tabba wrote: > > [..] > > kvmtool closer to a more consistent way of allocating guest memory, in > > a similar manner to other VMMs. > > I would really appreciate pointing me to where qemu allocates memory using > memfd when invoked with -m <size>. I was able to follow the hostmem-ram > backend allocation function until g_malloc0(), but I couldn't find the > implementation for that. As far as I can tell, qemu allocates memory without backing storage (so by specifying only -m on the command line) like this: main -> qemu_init -> qmp_x_exit_preconfig -> qemu_init_board -> create_default_memdev, which creates a TYPE_MEMORY_BACKEND_RAM object. When creating the VM ram, the object's alloc function is called in: create_default_memdev -> user_creatable_complete -> host_memory_backend_complete) -> ram_backend_memory_alloc -> memory_region_init_ram_flags_nomigrate -> qemu_ram_alloc -> qemu_ram_alloc_internal -> ram_block_add -> qemu_anon_ram_alloc -> qemu_ram_mmap(fd=-1,..) -> mmap_activate(..,fd=-1,..) -> mmap(..,MAP_ANONYMOUS,fd=-1,..) Unless I'm mistaken with the above (it was quite convoluted to unwrap all of this), qemu doesn't allocate RAM for the VM using a backing file, unless specifically requested by the user. On the other hand. for crosvm: main -> crosvm_main -> run_vm -> run_config (from src/scrovm/sys/unix.rs), creates the memory layout in GuestMemory::new -> MemoryMappingBuilder::new -> from_shared_memory -> offset -> build. I couldn't find the implementation for MemoryMappingBuilder::build, if it's anything like build_fixed, then indeed it looks like it uses the memfd created with GuestMemory::create_shm and passed to MemoryMappingBuild::from_shared_offset. Thanks, Alex
