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hostmem: add a new memory backend based on POSIX shm_open()

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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.

Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com> (QAPI schema)
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20240618100519.145853-1-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Stefano Garzarella 2024-06-18 12:05:19 +02:00 • committed by Michael S. Tsirkin
parent 5ab04420c3
commit 4e647fa085
5 changed files with 164 additions and 4 deletions
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5
docs/system/devices/vhost-user.rst
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@ -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