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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] question about running vm change its mem maxsize
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:hiI 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 VMIn 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"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?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 notchanging the maximum vm size, but allocating the initial number of pages required to load the guest operating system image into. that's typicallymuch less than d->max_pages.I am confused about itis 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 thatset_vram_mapping()->set_mm_mapping()->xc_domain_memory_populate_physmap() ,are there any other call path in the xen code? I only find this one in the paravirt case,VM does not manage the whole physical memory it could use (say 512M for maxsize,or something)when vm boot up,instead,xen dynamically allocate some memory(both virtual and physical )to VM,when VM requests, while raditional linux manage its physical memory using mem_map or so when it boot up ,while allocates virtual memory dynamically,is it right? I am confused about it 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.I see it is domain0 who has the control interface to set and change the max-size of the VM memorythen 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.when VM is running, we can "xm mem-set " or "xm mem-max "in the domain0'console to change the vm memoryI have used it , and want to know what happen in the xen ,espacially in a paravirtVMwhen 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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