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 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
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? 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?
could you help me
thanks in advance



> lets say you build a domain of size 256MB. that domain initially won't
> need the whole 256MB to run. what it initially needs is memory where the
> kernel is loaded. lets say that's 4MB or something. as soon as the guest
> kernel is running, it will allocate any additionally needed memory by
> itself and some magic mentioned in my previous replay. 
>
> it won't get more than those 256MB, unless root on dom0 is willing to
> say so. it will learn about it, in a similar fashion to which a native
> operating system gathers information about installed hardware from the
> bios. that's the max_pages variable above.
>
> before, domain0 will perform as a the boot loader, responsible for
> loading the kernel image into main memory of the virtual machine, setup
> a virtual cpu to aim at the kernels entry point, prepare virtual I/O
> devices and whatever else it sees fit, and then fire up the domain.
>
> in order to install the kernel, some of the guest vm memory must be
> available. max_pages just says how much it could be, presently it's
> still zero. the domain builder needs in our case the lets-say-4MB
> mentioned above, at specific positions in the guest memory map, to copy
> the kernel image into it (e.g. linux is typically mapped at adresses
> above 1M). 
> that is what xc_domain_memory_populate_physmap() is doing.
> while xen allocates the memory, it won't load the kernel image. that's
> way better done in user space. for that purpose, and about a ton of
> others, dom0 is privileged to mmap() the page frames of arbitrary other
> domains. if you follow the code, you will see calls to
> xc_map_foreign_range(). e.g. xc_load_elf.c:loadelfimage().
>
> but, in order to map these pages, the domain builder needs to know which
> pages xen actually allocated. pages are numbered by the upper bits of
> their location in physical memory. physical memory (as opposed to
> per-domain 'pseudo-physical' memory, is identified by machine frame
> numbers (mfn). the page_table argument to populate_physmap is requesting
> exactly these.
>
> one mfn is an unsigned long. for a domain with size nr_pages, you need
> nr_pages * sizeof(unsigned long) bytes to hold the mfn table for the
> domain. 
>
>   
> > > > 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
> >     
> when max_pages is set, there's not much happening initially, as said
> before. the domain is allowed to allocate more memory. that memory
> remains unallocated up to the point where the domain actually allocates
> it.
>
> i'm not entirely sure about how this is currently mapped in xen to
> physical memory. i believe max_pages may be potentially different to the
> amount of phyiscal memory announced to the domain. alternatives might
> include memory hotplugging apis in the guest operating system. maybe
> someone else can comment.
>
> regards,
> daniel
>
>
>   

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