Re: [PATCH kvmtool v1 08/17] Use memfd for all guest ram allocations

[Alexandru Elisei <alexandru.elisei@arm.com>]

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