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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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16
qemu-options.hx
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@ -5240,6 +5240,22 @@ SRST
The ``share`` boolean option is on by default with memfd.
``-object memory-backend-shm,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
Creates a POSIX shared memory backend object, which allows
QEMU to share the memory with an external process (e.g. when
using vhost-user).
``memory-backend-shm`` is a more portable and less featureful version
of ``memory-backend-memfd``. It can then be used in any POSIX system,
especially when memfd is not supported.
Please refer to ``memory-backend-file`` for a description of the
options.
The ``share`` boolean option is on by default with shm. Setting it to
off will cause a failure during allocation because it is not supported
by this backend.
``-object iommufd,id=id[,fd=fd]``
Creates an iommufd backend which allows control of DMA mapping
through the ``/dev/iommu`` device.