On 7/29/2024 8:29 AM, Igor Mammedov wrote:
On Sat, 20 Jul 2024 16:28:25 -0400
Steven Sistare <steven.sist...@oracle.com> wrote:
On 7/16/2024 5:19 AM, Igor Mammedov wrote:
On Sun, 30 Jun 2024 12:40:24 -0700
Steve Sistare <steven.sist...@oracle.com> wrote:
Allocate anonymous memory using mmap MAP_ANON or memfd_create depending
on the value of the anon-alloc machine property. This affects
memory-backend-ram objects, guest RAM created with the global -m option
but without an associated memory-backend object and without the -mem-path
option
nowadays, all machines were converted to use memory backend for VM RAM.
so -m option implicitly creates memory-backend object,
which will be either MEMORY_BACKEND_FILE if -mem-path present
or MEMORY_BACKEND_RAM otherwise.
Yes. I dropped an an important adjective, "implicit".
"guest RAM created with the global -m option but without an explicit
associated
memory-backend object and without the -mem-path option"
To access the same memory in the old and new QEMU processes, the memory
must be mapped shared. Therefore, the implementation always sets
RAM_SHARED if alloc-anon=memfd, except for memory-backend-ram, where the
user must explicitly specify the share option. In lieu of defining a new
so statement at the top that memory-backend-ram is affected is not
really valid?
memory-backend-ram is affected by alloc-anon. But in addition, the user must
explicitly add the "share" option. I don't implicitly set share in this case,
because I would be overriding the user's specification of the memory object's
property,
which would be private if omitted.
instead of touching implicit RAM (-m), it would be better to error out
and ask user to provide properly configured memory-backend explicitly.
RAM flag, at the lowest level the implementation uses RAM_SHARED with fd=-1
as the condition for calling memfd_create.
In general I do dislike adding yet another option that will affect
guest RAM allocation (memory-backends should be sufficient).
However I do see that you need memfd for device memory (vram, roms, ...).
Can we just use memfd/shared unconditionally for those and
avoid introducing a new confusing option?
The Linux kernel has different tunables for backing memfd's with huge pages, so
we
could hurt performance if we unconditionally change to memfd. The user should
have
a choice for any segment that is large enough for huge pages to improve
performance,
which potentially is any memory-backend-object. The non memory-backend objects
are
small, and it would be OK to use memfd unconditionally for them.
Thanks everyone for your feedback. The common theme is that you dislike that
the
new option modifies the allocation of memory-backend-objects. OK, accepted. I
propose
to remove that interaction, and document in the QAPI which backends work for
CPR.
Specifically, memory-backend-memfd or memory-backend-file object is required,
with share=on (which is the default for memory-backend-memfd). CPR will be
blocked
otherwise. The legacy -m option without an explicit memory-backend-object will
not
support CPR.
Non memory-backend-objects (ramblocks not described on the qemu command line)
will always
be allocated using memfd_create (on Linux only). The alloc-anon option is
deleted.
The logic in ram_block_add becomes:
if (!new_block->host) {
if (xen_enabled()) {
...
} else if (!object_dynamic_cast(new_block->mr->parent_obj.parent,
TYPE_MEMORY_BACKEND)) {
qemu_memfd_create()
} else {
qemu_anon_ram_alloc()
}
Is that acceptable to everyone? Igor, Peter, Daniel?
- Steve
