Re: [Qemu-devel] qemu memory manage question

[Peter Xu]

On Mon, Apr 17, 2017 at 06:09:11PM +0800, jack.chen wrote:
> hello,I have some questions about memory allocation in qemu for
> virtual machine.I found when configure_accelerator function was called
> ,memory slots  were registered to KVM,but at that time address_space
> have not been initialized and ram have not been allocated,it is really
> confused me,Thanks a lot!!

Here's how I understand it...

configure_accelerator() does not register memory slots in KVM.
Instead, it registers memory listeners. See
kvm_memory_listener_register(), especially:

    kml->listener.region_add = kvm_region_add;

That's the hook function to be called when there are new memory region
added to the system.

Further, when RAM is initialzed, it'll modify the address space layout
of system_memory, and the registered listener of KVM (kvm_region_add)
will be invoked, it'll further sync with kvm. It should be in the
following path if you break at kvm_region_add in gdb:

#0  0x00005555557ba13a in kvm_region_add (listener=0x5555568330c0, 
section=0x7fffffffd310) at /root/git/qemu/kvm-all.c:859
#1  0x00005555557c1910 in address_space_update_topology_pass (as=0x55555629e240 
<address_space_memory>, old_view=0x5555567a7090, new_view=0x5555568d3460, 
adding=true) at /root/git/qemu/memory.c:871
#2  0x00005555557c19f3 in address_space_update_topology (as=0x55555629e240 
<address_space_memory>) at /root/git/qemu/memory.c:886
#3  0x00005555557c1b41 in memory_region_transaction_commit () at 
/root/git/qemu/memory.c:922
#4  0x00005555557c4bfd in memory_region_update_container_subregions 
(subregion=0x5555568d2fc0) at /root/git/qemu/memory.c:2075
#5  0x00005555557c4c64 in memory_region_add_subregion_common 
(mr=0x5555567a5830, offset=0, subregion=0x5555568d2fc0) at 
/root/git/qemu/memory.c:2085
#6  0x00005555557c4ca0 in memory_region_add_subregion (mr=0x5555567a5830, 
offset=0, subregion=0x5555568d2fc0) at /root/git/qemu/memory.c:2093
#7  0x000055555583fd68 in pc_memory_init (pcms=0x5555567a4100, 
system_memory=0x5555567a5830, rom_memory=0x5555568d21a0, 
ram_memory=0x7fffffffd550) at /root/git/qemu/hw/i386/pc.c:1383
#8  0x0000555555847363 in pc_q35_init (machine=0x5555567a4100) at 
/root/git/qemu/hw/i386/pc_q35.c:147
#9  0x0000555555847cac in pc_init_v2_9 (machine=0x5555567a4100) at 
/root/git/qemu/hw/i386/pc_q35.c:310
#10 0x00005555558f7cf8 in main (argc=11, argv=0x7fffffffda78, 
envp=0x7fffffffdad8) at /root/git/qemu/vl.c:4557

Hope this helps. Thanks.

-- 
Peter Xu