Re: [PATCH 9/9] hostmem-file: support POSIX shm_open()

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Feb 28, 2024 at 12:47:59PM +0100, Stefano Garzarella wrote:
> Add a new `shm` bool option for `-object memory-backend-file`.
> 
> When this option is set to true, the POSIX shm_open(3) is used instead
> of open(2).
> 
> So a file will not be created in the filesystem, but a "POSIX shared
> memory object" will be instantiated. In Linux this turns into a file
> in /dev/shm, but in other OSes this may not happen (for example in
> macOS or FreeBSD nothing is shown in any filesystem).
> 
> This new feature is useful when we need to share guest memory with
> another process (e.g. vhost-user backend), but we don't have
> memfd_create() or any special filesystems (e.g. /dev/shm) available
> as in macOS.
> 
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> I am not sure this is the best way to support shm_open() in QEMU.
> 
> Other solutions I had in mind were:
> 
> - create a new memory-backend-shm
> 
> - extend memory-backend-memfd to use shm_open() on systems where memfd is
> not available (problem: shm_open wants a name to assign to the object, but
> we can do a workaround by using a random name and do the unlink right away)

IMHO, create a new memory-backend-shm, don't overload memory-backend-memfd,
as this lets users choose between shm & memfd, even on Linux.

With regards,
Daniel
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