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

[Daniel Stodden <stodden@xxxxxxxxxx>]

On Tue, 2007-03-20 at 12:12 +0800, tgh wrote:
> Thank you for your reply
> 
> 
> Daniel Stodden åé:
> > On Mon, 2007-03-19 at 20:21 +0800, tgh wrote:
> >   
> >> 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
> >>     
> >
> > is see, i'm not sure anymore whether i understand your problem
> > correctly. 
> >
> > so lets try going through that more slowly.
> >
> > not the malloc() above, but the call to
> > xc_domain_memory_populate_physmap() allocates memory. domain memory is
> > organized in pages. those pages are allocated upon demand by the
> > software using it.
> >   
> I search the code ,it seems that
>  set_vram_mapping()->set_mm_mapping()->xc_domain_memory_populate_physmap()
> 
> 
> and domain builder acts as bootloader ,and then guestos-linux  will  
> setup and control its own memory ,is it right? of course,any memory-map 
> will incur xen to related map or so.
> but in the rational linux, it will know how many physical memory it owns 
> and linux know that all the physical memory it owns is there

naturally.

> In the xen ,guest-linux know how many physical memory it owns but 
> doesnot know that all the physical memory it owns is there,for some of 
> them is not to allocate,is it right?

this is correct -- for a paravirtual guest. for a hvm guest, it needs to
work a little different, of course.

>  that is  during guest-linux 
> running,it  will  request  some memory  which  it will  get ,for the 
> guest ,its memory (both its physical memory and its virtual memory)is 
> dynamical allocated,while raditional linux has its physical memory when 
> it boot up ,while gets virtual memory dynamically,is it right?

yes. see the xen interface manual on this topic, namely the chapters on
memory management and the hypercall reference for page table and
physical memory management. not sure if this gets entirely clear,
though, i sometimes tend to get confused myself. especially guest side
vm tends to be complex, but that's quite natural.

> could you help me
> thanks in advance

hth,
dns

-- 
Daniel Stodden
LRR     -      Lehrstuhl fÃr Rechnertechnik und Rechnerorganisation
Institut fÃr Informatik der TU MÃnchen             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel