On 11/4/2024 5:39 AM, David Hildenbrand wrote:
On 01.11.24 14:47, Steve Sistare wrote:
Allocate anonymous memory using mmap MAP_ANON or memfd_create depending
on the value of the anon-alloc machine property. This option applies to
memory allocated as a side effect of creating various devices. It does
not apply to memory-backend-objects, whether explicitly specified on
the command line, or implicitly created by the -m command line option.
The memfd option is intended to support new migration modes, in which the
memory region can be transferred in place to a new QEMU process, by sending
the memfd file descriptor to the process. Memory contents are preserved,
and if the mode also transfers device descriptors, then pages that are
locked in memory for DMA remain locked. This behavior is a pre-requisite
for supporting vfio, vdpa, and iommufd devices with the new modes.
A more portable, non-Linux specific variant of this will be using shm,
similar to backends/hostmem-shm.c.
Likely we should be using that instead of memfd, or try hiding the
details. See below.
For this series I would prefer to use memfd and hide the details. It's a
concise (and well tested) solution albeit linux only. The code you supply
for posix shm would be a good follow on patch to support other unices.
We could drop
-machine anon-alloc=mmap|memfd
and define
-machine anon-shared
as you suggest at the end.
[...]
@@ -69,6 +70,8 @@
#include "qemu/pmem.h"
+#include "qapi/qapi-types-migration.h"
+#include "migration/options.h"
#include "migration/vmstate.h"
#include "qemu/range.h"
@@ -1849,6 +1852,35 @@ static void ram_block_add(RAMBlock *new_block, Error
**errp)
qemu_mutex_unlock_ramlist();
return;
}
+
+ } else if (current_machine->anon_alloc == ANON_ALLOC_OPTION_MEMFD &&
+ !object_dynamic_cast(new_block->mr->parent_obj.parent,
+ TYPE_MEMORY_BACKEND)) {
This looks a bit and hackish,
OK. I can revert parts of the previous version which passed in RAM_SHARED from
various call sites to request anonymous shared memory:
https://lore.kernel.org/qemu-devel/1714406135-451286-18-git-send-email-steven.sist...@oracle.com
See the various sites that do
uint32_t flags = current_machine->memfd_alloc ? RAM_SHARED : 0;
Does that look OK to you?
and I don't think ram_block_add() is the right
place where this should be. It should likely happen in the caller.
I agree, but I received no feedback when I proposed to refactor allocation
vs ram_block_add, so I dropped them to simplify the live update review.
These refactor but do not change functionality. Are you OK with something
like this? Is this overkill?
https://lore.kernel.org/qemu-devel/1714406135-451286-1-git-send-email-steven.sist...@oracle.com/
physmem: ram_block_create
physmem: hoist guest_memfd creation
physmem: hoist host memory allocation
We already do have two ways of allocating "shared anonymous memory":
(1) memory-backend-ram,share=on
(2) memory-backend-shm
(2) gives us an fd as it uses shm_open(), (1) doesn't give us an fd as it
uses MAP_ANON|MAP_SHARED. (1) is really only a corner case use case [1].
[there is also Linux specific memfd, which gives us more flexibility with
hugetlb etc, but for the purpose here shm should likely be sufficient?]
So why not make (1) behave like (2) and move that handling into
qemu_ram_alloc_internal(), from where we can easily enable it using a
new RMA_SHARED flag? So as a first step, something like:
I prefer that, and an earlier version did so, but only if anon-alloc==memfd.
To be clear, do you propose that memory-backend-ram,shared=on unconditionally
mmap fd-based shared memory, independently of the setting of anon-alloc?
And drop the MAP_ANON|MAP_SHARED possibility?
Or, do you propose that for memory-backend-ram,shared=on:
if anon-shared
mmap fd
else
MAP_ANON|MAP_SHARED
The former is simpler from a user documentation point of view, but either
works for me. I could stop listing memory-backend-ram as an exception in
the docs, which currently state:
# Memory-backend objects must have the share=on attribute, but
# memory-backend-epc and memory-backend-ram are not supported.
[...]
Then, you only need a machine option to say "anon-shared", to make all
anonymous memory sharable between processes. All it would do is setting
the RAM_SHARED flag in qemu_ram_alloc_internal() when reasonable
(!(ram_flags & RAM_PREALLOC)).
To handle "memory-backend-ram,share=off", can we find a way to bail out if
memory-backend-ram,share=off was used while the machine option "anon-shared"
would be active?
In later patches I install migration blockers for various conditions, including
when a ram block does not support CPR.
Or just document that the "anon-shared" will win?
IMO a blocker is sufficient.
I think you are also suggesting that an unadorned "memory-backend-ram"
specification (with implicit shared=off), plus anon-shared, should cause
shared anon to be allocated:
"you only need a machine option to say "anon-shared", to make all anonymous
memory sharable"
I did that previously, and Peter objected, saying the explicit anon-shared
should not override the implicit shared=off.
But perhaps I misinterpret someone.
- Steve
Alternatives might be a RAM_PFORCE_PRIVATE flag, set by the memory backend.
With above change, we could drop the "bool share" flag from,
qemu_anon_ram_alloc(), as it would be unused.
[1]
https://patchwork.kernel.org/project/qemu-devel/patch/20180201205511.19198-2-mar...@redhat.com/
