Re: [Qemu-devel] QEMU patch to allow VM introspection via libvmi

[Valerio Aimale]

On 10/26/15 11:52 AM, Eduardo Habkost wrote:
> I was trying to advocate the use of a shared mmap'ed region. The sharing
> would be two-ways (RW for both) between the QEMU virtualizer and the libvmi
> process. I envision that there could be a QEMU command line argument, such
> as "--mmap-guest-memory <filename>" Understand that Eric feels strongly the
> libvmi client should own the file name - I have not forgotten that. When
> that command line argument is given, as part of the guest initialization,
> QEMU creates a file of size equal to the size of the guest memory containing
> all zeros, mmaps that file to the guest memory with  PROT_READ|PROT_WRITE
> and MAP_FILE|MAP_SHARED, then starts the guest.
> This is basically what memory-backend-file (and the legacy -mem-path
> option) already does today, but it unlinks the file just after opening
> it. We can change it to accept a full filename and/or an option to make
> it not unlink the file after opening it.
>
> I don't remember if memory-backend-file is usable without -numa, but we
> could make it possible somehow.
Eduardo, I did try this approach. It takes 2 line changes in exec.c: 
comment the unlink out, and making sure MAP_SHARED is used when 
-mem-path and -mem-prealloc are given. It works beautifully, and libvmi 
accesses are fast. However, the VM is slowed down to a crawl, obviously, 
because each RAM access by the VM triggers a page fault on the mmapped 
file. I don't think having a crawling VM is desirable, so this approach 
goes out the door.

I think we're back at estimating the speed of other approaches as 
discussed previously:

- via UNIX socket as per existing patch
- via xp parsing the human readable xp output
- via an xp-like command the returns memory content baseXX-encoded into 
a json string
- via shared memory as per existing code and patch

Any other?