Re: [PATCH v6 10/12] hostmem: add a new memory backend based on POSIX shm_open()

[Markus Armbruster]

Stefano Garzarella <sgarzare@redhat.com> writes:

> On Wed, May 29, 2024 at 04:50:20PM GMT, Markus Armbruster wrote:
>>Stefano Garzarella <sgarzare@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 <david@redhat.com>
>>> Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
>>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>> ---
>>> v5
>>> - fixed documentation in qapi/qom.json and qemu-options.hx [Markus]
>>> 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                      |  19 +++++
>>>  backends/hostmem-shm.c             | 123 +++++++++++++++++++++++++++++
>>>  backends/meson.build               |   1 +
>>>  qemu-options.hx                    |  16 ++++
>>>  5 files changed, 162 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..d40592d863 100644
>>> --- a/qapi/qom.json
>>> +++ b/qapi/qom.json
>>> @@ -721,6 +721,21 @@
>>>              '*hugetlbsize': 'size',
>>>              '*seal': 'bool' } }
>>>
>>> +##
>>> +# @MemoryBackendShmProperties:
>>> +#
>>> +# Properties for memory-backend-shm objects.
>>> +#
>>> +# The @share boolean option is true by default with shm. Setting it to 
>>> false
>>> +# will cause a failure during allocation because it is not supported by 
>>> this
>>> +# backend.
>>
>>docs/devel/qapi-code-gen.rst:
>>
>>    For legibility, wrap text paragraphs so every line is at most 70
>>    characters long.
>>
>>    Separate sentences with two spaces.
>>
>>Result:
>>
>>   # Properties for memory-backend-shm objects.
>>   #
>>   # The @share boolean option is true by default with shm.  Setting it
>>   # to false will cause a failure during allocation because it is not
>>   # supported by this backend.
>
> Ops, sorry, I'll fix!
>
>>
>>However, 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.
>
> Yes, I had already seen your comment to an earlier version and sent another 
> separate patch:
> https://patchew.org/QEMU/20240523133302.103858-1-sgarzare@redhat.com/
>
> Is that okay?

Looks like I'm going through my post-vacation review backlog in
suboptimal order...

Replied there!

>>> +#
>>> +# Since: 9.1
>>> +##
>>> +{ 'struct': 'MemoryBackendShmProperties',
>>> +  'base': 'MemoryBackendProperties',
>>> +  'data': { } }
>>
>>Let's add 'if': 'CONFIG_POSIX' here.
>>
>
> I think my response to your review at v4 fell through a crack :-)
> https://patchew.org/QEMU/20240508074457.12367-1-sgarzare@redhat.com/20240508074457.12367-11-sgarzare@redhat.com/#z3lbtmkn6zlwdhdea7owav3mblttxr3asrmlilwxmkla67tdby@732gn3uuupoq

Dang, it did %-}

> I'll bring back my doubts here:
>
>   Do you mean something like this:
>
>   { 'struct': 'MemoryBackendShmProperties',
>      'if': 'CONFIG_POSIX',
>      'base': 'MemoryBackendProperties',
>      'data': { } }
>
>   I didn't because for MemoryBackendMemfdProperties and
>   MemoryBackendEpcProperties we have 'if': 'CONFIG_POSIX' only later in
>   the ObjectOptions union, so I did the same.
>
>   Should we fix them as well?

Yes, please.

The QAPI schema's primary purpose is to define the QMP interface.  The
tooling lets you define QAPI types that aren't actually used in the QMP
interface.  We use this intentionally, e.g. to generate types & visitors
for complex QOM properties.  Accidental use is also possible, say when
we define a type unconditionally, but use it only conditionally.  We
then end up generating dead code.  No big deal, but let's avoid it
whenever practical.

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