On Wed, Feb 28, 2024 at 12:08:37PM +0000, Daniel P. Berrangé wrote:
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 <sgarz...@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.

Yeah, good point!
I think there's enough of a consensus on adding memory-backend-shm, so I'm going to go toward that direction in v2.


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