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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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23
qapi/qom.json
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@ -601,8 +601,8 @@
#
# @share: if false, the memory is private to QEMU; if true, it is
# shared (default false for backends memory-backend-file and
# memory-backend-ram, true for backends memory-backend-epc and
# memory-backend-memfd)
# memory-backend-ram, true for backends memory-backend-epc,
# memory-backend-memfd, and memory-backend-shm)
#
# @reserve: if true, reserve swap space (or huge pages) if applicable
# (default: true) (since 6.1)
@ -721,6 +721,21 @@
'*hugetlbsize': 'size',
'*seal': 'bool' } }
##
# @MemoryBackendShmProperties:
#
# Properties for memory-backend-shm objects.
#
# This memory backend supports only shared memory, which is the
# default.
#
# Since: 9.1
##
{ 'struct': 'MemoryBackendShmProperties',
'base': 'MemoryBackendProperties',
'data': { },
'if': 'CONFIG_POSIX' }
##
# @MemoryBackendEpcProperties:
#
@ -1049,6 +1064,8 @@
{ 'name': 'memory-backend-memfd',
'if': 'CONFIG_LINUX' },
'memory-backend-ram',
{ 'name': 'memory-backend-shm',
'if': 'CONFIG_POSIX' },
'pef-guest',
{ 'name': 'pr-manager-helper',
'if': 'CONFIG_LINUX' },
@ -1121,6 +1138,8 @@
'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties',
'if': 'CONFIG_LINUX' },
'memory-backend-ram': 'MemoryBackendProperties',
'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
'if': 'CONFIG_POSIX' },
'pr-manager-helper': { 'type': 'PrManagerHelperProperties',
'if': 'CONFIG_LINUX' },
'qtest': 'QtestProperties',