Re: Share host memory with a guest running no operating system

[Jakob Bohm]

On 2022-06-21 03:38, John Ramsden wrote:

> I'm attempting to share host memory with a guest running no operating system using `-object memory-backend-file,id=mem0,size=4G,mem-path=/dev/shm/qemu,share=on`. When I try this using a guest with an operating system such as Ubuntu, I see my object mem0 in `info mtree`, and I'm able to write to `/dev/shm/qemu` on the host and read the changed memory on the VM - similar to this blog post (I am not the author) https://blog.reds.ch/?p=1379. This doesn't seem to work when I have a guest without an operating system, there is no `mem0` device. In case it's relevant I am using an arm64 guest.
>
> Do I need a guest operating system to use this feature?
>
>     
    It is important to understand the difference
      between the two qemu variants available
      for each architecture:
      
      In the qemu-variant named qemu-archname-system, your guest runs as
      bare metal program
      (like memtest86) that can be run on a computer without booting an
      operating system,
      there are no device or file names in the guest, only hardware
      addresses that access
      the various devices.  Essentially, such programs are their own
      primitive OS.
      
      In the qemu-variant named just qemu-archname, your guest is a
      Linux application
      compiled for the wrong CPU, it sees exactly the same device names
      and files as
      the host machine, as that qemu program only simulates the CPU
      instruction set,
      not an entire machine, device simulation options make no sense for
      that
      qemu variant.

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Jakob Bohm, CIO, partner, WiseMo A/S. https://www.wisemo.com
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