On 08.04.24 09:58, Stefano Garzarella wrote:
On Thu, Apr 04, 2024 at 04:09:34PM +0200, David Hildenbrand wrote:
On 04.04.24 14:23, Stefano Garzarella wrote:
shm_open() creates and opens a new POSIX shared memory object.
A POSIX shared memory object allows creating memory backend with an
associated file descriptor that can be shared with external processes
(e.g. vhost-user).
The new `memory-backend-shm` can be used as an alternative when
`memory-backend-memfd` is not available (Linux only), since shm_open()
should be provided by any POSIX-compliant operating system.
This backend mimics memfd, allocating memory that is practically
anonymous. In theory shm_open() requires a name, but this is allocated
for a short time interval and shm_unlink() is called right after
shm_open(). After that, only fd is shared with external processes
(e.g., vhost-user) as if it were associated with anonymous memory.
In the future we may also allow the user to specify the name to be
passed to shm_open(), but for now we keep the backend simple, mimicking
anonymous memory such as memfd.
Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
---
v3
- enriched commit message and documentation to highlight that we
want to mimic memfd (David)
---
docs/system/devices/vhost-user.rst | 5 +-
qapi/qom.json | 17 +++++
backends/hostmem-shm.c | 118 +++++++++++++++++++++++++++++
backends/meson.build | 1 +
qemu-options.hx | 11 +++
5 files changed, 150 insertions(+), 2 deletions(-)
create mode 100644 backends/hostmem-shm.c
diff --git a/docs/system/devices/vhost-user.rst
b/docs/system/devices/vhost-user.rst
index 9b2da106ce..35259d8ec7 100644
--- a/docs/system/devices/vhost-user.rst
+++ b/docs/system/devices/vhost-user.rst
@@ -98,8 +98,9 @@ Shared memory object
In order for the daemon to access the VirtIO queues to process the
requests it needs access to the guest's address space. This is
-achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
-objects. A reference to a file-descriptor which can access this object
+achieved via the ``memory-backend-file``, ``memory-backend-memfd``, or
+``memory-backend-shm`` objects.
+A reference to a file-descriptor which can access this object
will be passed via the socket as part of the protocol negotiation.
Currently the shared memory object needs to match the size of the main
diff --git a/qapi/qom.json b/qapi/qom.json
index 85e6b4f84a..5252ec69e3 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -721,6 +721,19 @@
'*hugetlbsize': 'size',
'*seal': 'bool' } }
+##
+# @MemoryBackendShmProperties:
+#
+# Properties for memory-backend-shm objects.
+#
+# The @share boolean option is true by default with shm.
+#
+# Since: 9.1
+##
+{ 'struct': 'MemoryBackendShmProperties',
+ 'base': 'MemoryBackendProperties',
+ 'data': { } }
+
Acked-by: David Hildenbrand <da...@redhat.com>
One comment: we should maybe just forbid setting share=off. it doesn't
make any sense and it can even result in an unexpected double memory
consumption. We missed doing that for memfd, unfortunately.
Good point!
IIUC the `share` property is defined by the parent `hostmem`, so I
should find a way to override the property here and disable the setter,
or add an option to `hostmem` to make the property non-writable.
Right, or simply fail later when you would find "share=off" in
shm_backend_memory_alloc().
When ever supporting named shmem_open(), it could make sense for VM
snapshotting. Right now it doesn't really make any sense.
--
Cheers,
David / dhildenb
