Re: [Xen-devel] question about running vm change its mem maxsize

[tgh <tianguanhua@xxxxxxxxxx>]

Thank you for your reply

Daniel Stodden åé:
> On Mon, 2007-03-19 at 09:20 +0800, tgh wrote:
>   
> > hi
> >  I  read the code of xc_linux_build() and xc_domain_setmaxmem(),and I am 
> > confused about how does "xm mem-max" change the  max  size  of mem  for  
> > a  running  VM
> >  In xc_domain_setmaxmem() ,the XEN_DOMCTL_max_mem has been called ,and  
> > just  does  "d->max_pages = new_max;"
> >     
> that variable determines the maximum size. the code verifies that the
> new size won't be below the previous one, and therefore just needs to
> readjust it.
>
> it doesn't actually have to allocate memory. this is done on demand,
> i.e. as soon as the domain references a new page frame within it's
> virtual machine address space.
>
>   
I see
> > while in the xc_linux_build(),  before the vm boots up,the maxsize pfn 
> > is alloced in an array with fixsize, page_array = malloc(nr_pages * 
> > sizeof(unsigned long))
> >     
> i suppose you misunderstood what that call really does. it's not
> changing the maximum vm size, but allocating the initial number of pages
> required to load the guest operating system image into. that's typically
> much less than d->max_pages.
>
>   
"page_array = malloc(nr_pages *sizeof(unsigned long))" in xc_linux_build() is 
not to allocate the physical memory to the VM,then which code or function allocate the 
phy-mem to the VM?

I am  confused about it
> > then how does  a running vm  changes its mem maxsize ,especially for 
> > expanding its mem maxsize?
> >     
> for an unprivileged guest os, there is no such call. similar in the way
> your desktop machine (hopefully) won't call dell ordering more RAM
> without your knowledge.
>
> changing machine size is an administrative operation. it is defined
> during VM creating, and potential subject to refinement by the
> administrator on dom0.
>   
I see it is domain0 who has the control interface to set and change the 
max-size of the VM memory
when VM is running, we can "xm mem-set  " or "xm mem-max   "in the 
domain0'console to change the vm memory
I have used it , and want to know what happen in the xen   ,espacially  
in a paravirtVM
could you help me
Thanks in advance



> what the guest system does in order to get 'more memory' is, from the
> guest system point of view, more or less equivalent to what the guest
> operating system on a physical host would do: manage physical memory,
> find a free page frame, map it in one or more page tables, then start
> using it. xen detects that process of mapping this page frame, and
> performs the allocation. but, how this is done in detail largely depends
> on the type of VM (hardware-based, paravirtual, etc), and even then
> there are different variants.
>
> hth.
>
> regards,
> daniel
>
>   

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