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Tsirkin" , Igor Mammedov , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: [PATCH v4 10/12] hostmem: add a new memory backend based on POSIX shm_open() In-Reply-To: <20240508074457.12367-11-sgarzare@redhat.com> (Stefano Garzarella's message of "Wed, 8 May 2024 09:44:54 +0200") References: <20240508074457.12367-1-sgarzare@redhat.com> <20240508074457.12367-11-sgarzare@redhat.com> Date: Wed, 08 May 2024 13:59:33 +0200 Message-ID: <87y18kcy56.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -26 X-Spam_score: -2.7 X-Spam_bar: -- X-Spam_report: (-2.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.582, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Stefano Garzarella 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 > Signed-off-by: Stefano Garzarella > --- > 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.