Re: [Qemu-devel] How does qemu know the virtual memory of the guest os?

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Nov 28, 2014 at 04:17:10PM -0800, Jidong Xiao wrote:
> Hi,
> 
> I notice that Qemu supports dump virtual memory of Guest OS. As this
> page suggests:
> 
> ========
> http://doc.opensuse.org/products/draft/SLES/SLES-kvm_sd_draft/cha.qemu.monitor.html
> 
> To save the content of the virtual machine memory to a disk or console
> output, use the following commands:
> 
> memsave addr size filename
> 
> Saves virtual memory dump starting at addr of size size to file filename
> 
> pmemsave addr size filename
> 
> Saves physical memory dump starting at addr of size size to file filename
> =========
> 
> I understand that hypervisors certainly know the physical memory of
> virtual machine, but how does it know the virtual memory of the Guest
> OS? I think the hypervisor has no semantic knowledge of the Guest OS,
> and such knowledge should be different for different OS (e.g., Windows
> vs Linux), so I am really surprised that Qemu can dump the virtual
> memory of the Guest OS. Can someone kindly give me some explanation?
> Thank you very much!!

It's different for each *architecture*, but not for each OS.

For example on x86 it starts by reading the CR* control registers, and
then the page tables (see target-i386/helper.c:
x86_cpu_get_phys_page_debug).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org