From mboxrd@z Thu Jan  1 00:00:00 1970
From: Cam Macdonell <cam@cs.ualberta.ca>
Subject: Re: Sharing a page of memory between the guest and host
Date: Wed, 12 Mar 2008 17:36:18 -0600
Message-ID: <47D868F2.3050901@cs.ualberta.ca>
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Cc: kvm-devel <kvm-devel@lists.sourceforge.net>
To: Anthony Liguori <anthony@codemonkey.ws>
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Anthony Liguori wrote:
> 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.

Hi Anthony,

In setting up the guest memory to share a page, I need to make the 
kernel aware of that page.  Is the best way to do this through a little 
virtio-like PCI device or is there an easier to get dynamic info into 
the guest kernel?

Also, is the mem-file implementation in a specific branch of the latest 
GIT or is it within the main branch somewhere, I've been unable to find it?

Thanks a bunch,
Cam

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