Stefano Garzarella <sgarz...@redhat.com> writes:
> 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 <da...@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
> ---
> v4
> - fail if we find "share=off" in shm_backend_memory_alloc() [David]
> 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 | 123 +++++++++++++++++++++++++++++
> backends/meson.build | 1 +
> qemu-options.hx | 13 +++
> 5 files changed, 157 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 38dde6d785..52df052df8 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.
This contradicts the doc comment for @share:
# @share: if false, the memory is private to QEMU; if true, it is
# shared (default: false)
Your intention is to override that text. But that's less than clear.
Moreover, the documentation of @share is pretty far from this override.
John Snow is working on patches that'll pull it closer.
Hmm, MemoryBackendMemfdProperties has the same override.
I think we should change the doc comment for @share to something like
# @share: if false, the memory is private to QEMU; if true, it is
# shared (default depends on the backend type)
and then document the actual default with each backend type.
> +#
> +# Since: 9.1
> +##
> +{ 'struct': 'MemoryBackendShmProperties',
> + 'base': 'MemoryBackendProperties',
> + 'data': { } }
Let's add 'if': 'CONFIG_POSIX' here.
> +
> ##
> # @MemoryBackendEpcProperties:
> #
> @@ -985,6 +998,8 @@
> { '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' },
> @@ -1056,6 +1071,8 @@
> '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',
[...]
> diff --git a/qemu-options.hx b/qemu-options.hx
> index cf61f6b863..2226172fb0 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -5227,6 +5227,19 @@ 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). This backend mimics memfd, allocating memory that
> is
> + practically anonymous. This is useful when memfd is not available.
This actually explains the purpose, unlike the doc comment in qom.json.
Same for the existing memory backends; can't fault you for doing your
new one the same way. We ought to fix them all. I'm not demanding you
do it.
The text could perhaps a bit clearer. What does "practically anonymous"
mean? As far as I understand, memory-backend-shm is a more portable and
less featureful version of memory-backend-memfd. Say that?
> +
> + 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.
> +
The "will case a failure" part is documented only here, and not in
qom.json.
Not this patch's fault: documentation for -object memory-backend-epc is
missing.
> ``-object iommufd,id=id[,fd=fd]``
> Creates an iommufd backend which allows control of DMA mapping
> through the ``/dev/iommu`` device.
