On 23.07.25 14:45, David Hildenbrand wrote:
On 23.07.25 14:42, Albert Esteve wrote:
On Wed, Jul 23, 2025 at 2:32 PM Philippe Mathieu-Daudé
<phi...@linaro.org> wrote:
Hi,
On 23/7/25 14:19, Albert Esteve wrote:
In the last version of the SHMEM MAP/UNMAP [1] there was a
comment [2] from Stefan about the lifecycle of the memory
regions.
After some discussion, David Hildenbrand proposed
to detect RAM regions and handle refcounting differently
to clear the initial concern. This RFC patch is
meant for gathering feedback from others
(i.e., Paolo Bonzini and Peter Xu).
[1] https://patchwork.ozlabs.org/project/qemu-devel/list/?series=460121
[2] https://patchwork.ozlabs.org/comment/3528600/
---
This patch enhances memory_region_ref() and memory_region_unref()
to handle RAM and MMIO memory regions differently, improving
reference counting semantics.
RAM regions now reference/unreference the memory region object
itself, while MMIO continue to reference/unreference the owner
device as before.
An additional qtest has been added to stress the memory
lifecycle. All tests pass as these changes keep backward
compatibility (prior behaviour is kept for MMIO regions).
Signed-off-by: David Hildenbrand <da...@redhat.com >
Signed-off-by: Albert Esteve <aest...@redhat.com>
---
system/memory.c | 22 +++++++++++++----
tests/qtest/ivshmem-test.c | 50 ++++++++++++++++++++++++++++++++++++++
2 files changed, 67 insertions(+), 5 deletions(-)
diff --git a/system/memory.c b/system/memory.c
index 5646547940..48ab6e5592 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -1826,6 +1826,14 @@ Object *memory_region_owner(MemoryRegion *mr)
void memory_region_ref(MemoryRegion *mr)
{
+ /* Regions without an owner are considered static. */
+ if (!mr || !mr->owner) {
+ return;
+ }
+ if (mr->ram) {
+ object_ref(OBJECT(mr));
+ return;
+ }
/* MMIO callbacks most likely will access data that belongs
* to the owner, hence the need to ref/unref the owner whenever
* the memory region is in use.
@@ -1836,16 +1844,20 @@ void memory_region_ref(MemoryRegion *mr)
* Memory regions without an owner are supposed to never go away;
What are the use cases for MRs without QOM owner?
Not sure if you are asking about the logic or the actual usecases
where these MRs would make sense.
Regarding the logic, note the early return at the beginning of the
function, so that this comment is kept valid. In short, nothing
changes.
Regarding the usecases for these type of memories, I can think of
system memory or container regions as examples. But there are
certainly more experienced people in this thread that can answer you
better than me.
The thing is: these MRs have an owner, but to make the limitation
spelled out in the doc (see my comment) work, we must refcount the MR
itself.
We could likely ref both (RAM region and the owner), but it's documented
that that results in a performance problem.
Correction: we can't easily, because of the object_unparent IIRC.
--
Cheers,
David / dhildenb
