Re: Sharing a page of memory between the guest and host

[Anthony Liguori <anthony@codemonkey.ws>]

Cam Macdonell wrote:
> Hello,
>
> Is it possible to share a memory (a page perhaps) between the host and 
> guest?

Yes, the host always has access to all of the guests memory.  All of the 
virtio drivers depend on this fact.  With KVM, the userspace (in this 
case, QEMU), just tells the kernel about a virtual address region and 
the kernel uses that region of virtual memory for the guest's physical 
memory.  Whatever you (as userspace) maps into that region is totally up 
to you.

>   More precisely, could a host and guest share a memory-mapped 
> file?

It will be a lot easier once we have MMU notifiers upstream.  You'll be 
able to simply mmap(MAP_FIXED) a file into the guest's physical address 
space even while it's running.  For now, you have to setup these 
mappings before the VM starts.

>   If one were crazy enough to want to do this, where should they 
> look first?
>   

If you look at the -mem-file implementation in the latest git, you'll 
see that all the guest's memory can be an mmap()'d file.

Regards,

Anthony Liguori

> Thanks,
> Cam
>
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