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From: Stefano Garzarella <sgarzare@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org, "Jason Wang" <jasowang@redhat.com>,
	"Coiby Xu" <Coiby.Xu@gmail.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	qemu-block@nongnu.org, "Daniel P. Berrangé" <berrange@redhat.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	slp@redhat.com, "Philippe Mathieu-Daudé" <philmd@linaro.org>,
	"Brad Smith" <brad@comstyle.com>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Thomas Huth" <thuth@redhat.com>,
	"Eric Blake" <eblake@redhat.com>, "Kevin Wolf" <kwolf@redhat.com>,
	"Raphael Norwitz" <raphael@enfabrica.net>,
	gmaglione@redhat.com, "Laurent Vivier" <lvivier@redhat.com>,
	stefanha@redhat.com, "David Hildenbrand" <david@redhat.com>,
	"Hanna Reitz" <hreitz@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Igor Mammedov" <imammedo@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>
Subject: Re: [PATCH v4 10/12] hostmem: add a new memory backend based on POSIX shm_open()
Date: Fri, 10 May 2024 11:37:05 +0200	[thread overview]
Message-ID: <z3lbtmkn6zlwdhdea7owav3mblttxr3asrmlilwxmkla67tdby@732gn3uuupoq> (raw)
In-Reply-To: <87y18kcy56.fsf@pond.sub.org>

On Wed, May 08, 2024 at 01:59:33PM 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>
>> Signed-off-by: Stefano Garzarella <sgarzare@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.

Yep, I followed @MemoryBackendMemfdProperties and 
@MemoryBackendEpcProperties.

>
>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.

I agree on that, but I think we should do in a separate series/patch.
If you prefer, I can send that patch.

>
>> +#
>> +# Since: 9.1
>> +##
>> +{ 'struct': 'MemoryBackendShmProperties',
>> +  'base': 'MemoryBackendProperties',
>> +  'data': { } }
>
>Let's add 'if': 'CONFIG_POSIX' 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?

>> +
>>  ##
>>  # @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?

Yeah, I see, I'll fix it with your suggestion!

>
>> +
>> +        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.

I'll add it also in qom.json.

>
>Not this patch's fault: documentation for -object memory-backend-epc is
>missing.

Oh right, I don't know anything about it. Should we ping Sean 
Christopherson <sean.j.christopherson@intel.com> who added it?

Thanks,
Stefano

>
>>      ``-object iommufd,id=id[,fd=fd]``
>>          Creates an iommufd backend which allows control of DMA mapping
>>          through the ``/dev/iommu`` device.
>



  reply	other threads:[~2024-05-10  9:38 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-05-08  7:44 [PATCH v4 00/12] vhost-user: support any POSIX system (tested on macOS, FreeBSD, OpenBSD) Stefano Garzarella
2024-05-08  7:44 ` [PATCH v4 01/12] libvhost-user: set msg.msg_control to NULL when it is empty Stefano Garzarella
2024-05-08  8:57   ` Daniel P. Berrangé
2024-05-08  9:33     ` Stefano Garzarella
2024-05-08 10:23   ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 02/12] libvhost-user: fail vu_message_write() if sendmsg() is failing Stefano Garzarella
2024-05-08  8:59   ` Daniel P. Berrangé
2024-05-08 10:23   ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 03/12] libvhost-user: mask F_INFLIGHT_SHMFD if memfd is not supported Stefano Garzarella
2024-05-08 10:39   ` Philippe Mathieu-Daudé
2024-05-10  8:25     ` Stefano Garzarella
2024-05-08  7:44 ` [PATCH v4 04/12] vhost-user-server: do not set memory fd non-blocking Stefano Garzarella
2024-05-08  9:00   ` Daniel P. Berrangé
2024-05-08  7:44 ` [PATCH v4 05/12] contrib/vhost-user-blk: fix bind() using the right size of the address Stefano Garzarella
2024-05-08 10:25   ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 06/12] contrib/vhost-user-*: use QEMU bswap helper functions Stefano Garzarella
2024-05-08 10:13   ` Philippe Mathieu-Daudé
2024-05-08 10:25     ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 07/12] vhost-user: enable frontends on any POSIX system Stefano Garzarella
2024-05-08 10:26   ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 08/12] libvhost-user: enable it " Stefano Garzarella
2024-05-08 10:36   ` Philippe Mathieu-Daudé
2024-05-10  8:56     ` Stefano Garzarella
2024-05-10  9:56       ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 09/12] contrib/vhost-user-blk: " Stefano Garzarella
2024-05-08 10:32   ` Philippe Mathieu-Daudé
2024-05-10  9:02     ` Stefano Garzarella
2024-05-08  7:44 ` [PATCH v4 10/12] hostmem: add a new memory backend based on POSIX shm_open() Stefano Garzarella
2024-05-08 11:59   ` Markus Armbruster
2024-05-10  9:37     ` Stefano Garzarella [this message]
2024-05-08  7:44 ` [PATCH v4 11/12] tests/qtest/vhost-user-blk-test: use memory-backend-shm Stefano Garzarella
2024-05-10  5:57   ` Thomas Huth
2024-05-10 10:54   ` Philippe Mathieu-Daudé
2024-05-08  7:44 ` [PATCH v4 12/12] tests/qtest/vhost-user-test: add a test case for memory-backend-shm Stefano Garzarella
2024-05-10  5:58   ` Thomas Huth
2024-05-08 10:39 ` [PATCH v4 00/12] vhost-user: support any POSIX system (tested on macOS, FreeBSD, OpenBSD) Philippe Mathieu-Daudé
2024-05-09 19:20 ` Stefan Hajnoczi

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