Hi,
I was trying to migrate a VM(CentOS7) which started with 4G memory and hot
plugged 5 memslots with 1G each. So the VM has total of 9G memory and
trying to migrate fails in vhost_dev_init() on destination host
if (used_memslots >
hdev->vhost_ops->vhost_backend_memslots_limit(hdev)) {
error_report("vhost backend memory slots limit is less"
" than current number of present memory slots");
r = -1;
if (busyloop_timeout) {
goto fail_busyloop;
} else {
goto fail;
}
}
Debugged more into this and understood that, after vhost_dev_init()
used_memslots are 4
(gdb) show-vhost-dev-memory-regions 0x555556d27000
n_mem_sections:4
n_regions:4
Region0:
$101 = {
guest_phys_addr = 0x100000000, ; 1G above 4G
memory_size = 0x40000000,
userspace_addr = 0x7fffb4600000,
flags_padding = 0x0
}
Region1:
$102 = {
guest_phys_addr = 0xc0000, ; 768k-896k
memory_size = 0x20000,
userspace_addr = 0x7ffef4200000,
flags_padding = 0x0
}
Region2:
$103 = {
guest_phys_addr = 0x100000, ;;3G from 1M
memory_size = 0xbff00000,
userspace_addr = 0x7ffef4700000,
flags_padding = 0x0
}
Region3:
$104 = {
guest_phys_addr = 0x0,
memory_size = 0xa0000, ; First 640K; 0 - 640k;
userspace_addr = 0x7ffef4600000,
flags_padding = 0x0
}
(gdb) p used_memslots
$150 = 4
And later on it coalesced the VGA region of 128k at 0xc0000( 768-896k) upon
guest config write
with i440fx_update_memory_mappings(d);
(gdb) f 5
#5 0x000055555595b920 in i440fx_write_config (dev=0x5555572b8000,
address=90, val=51, len=1) at hw/pci-host/piix.c:168
168 i440fx_update_memory_mappings(d);
(gdb) l
163
164 /* XXX: implement SMRAM.D_LOCK */
165 pci_default_write_config(dev, address, val, len);
166 if (ranges_overlap(address, len, I440FX_PAM, I440FX_PAM_SIZE) ||
167 range_covers_byte(address, len, I440FX_SMRAM)) {
168 i440fx_update_memory_mappings(d);
169 }
170 }
(gdb) p used_memslots
$150 = 3
Since the destination QEMU does not know that the source host coalesced one
region, vhost_dev_init for kernel started with 4 slots and hot plugged 5
slots and then the vhost_dev_init for user fails since 9>8 is true.
if (used_memslots >
hdev->vhost_ops->vhost_backend_memslots_limit(hdev)) {
error_report("vhost backend memory slots limit is less"
" than current number of present memory slots");
r = -1;
if (busyloop_timeout) {
goto fail_busyloop;
} else {
goto fail;
}
}
Can you guys please answer my questions here:
1) Why is the VGA region of 128k coalesced on guest vcpu execution with
pci config write?
2) We have memslots limit of 8 and 3 are pre occupied with initial memory
and we can only use 4 memslots for hotplug instead of 5 to migrate a VM.
Is there any reason for this? We cannot hotplug 5 slots and migrate the VM.
Thanks much
Suresh.G
