Re: 9 TiB vm memory creation

[David Hildenbrand <david@redhat.com>]

On 14.02.22 13:36, Igor Mammedov wrote:
> On Mon, 14 Feb 2022 10:54:22 +0530 (IST)
> Ani Sinha <ani@anisinha.ca> wrote:
> 
>> Hi Igor:
>>
>> I failed to spawn a 9 Tib VM. The max I could do was a 2 TiB vm on my
>> system with the following commandline before either the system
>> destabilized or the OOM killed killed qemu
>>
>> -m 2T,maxmem=9T,slots=1 \
>> -object memory-backend-file,id=mem0,size=2T,mem-path=/data/temp/memfile,prealloc=off \
>> -machine memory-backend=mem0 \
>> -chardev file,path=/tmp/debugcon2.txt,id=debugcon \
>> -device isa-debugcon,iobase=0x402,chardev=debugcon \
>>
>> I have attached the debugcon output from 2 TiB vm.
>> Is there any other commandline parameters or options I should try?
>>
>> thanks
>> ani
> 
> $ truncate -s 9T 9tb_sparse_disk.img
> $ qemu-system-x86_64 -m 9T \
>   -object memory-backend-file,id=mem0,size=9T,mem-path=9tb_sparse_disk.img,prealloc=off,share=on \
>   -machine memory-backend=mem0
> 
> works for me till GRUB menu, with sufficient guest kernel
> persuasion (i.e. CLI limit ram size to something reasonable) you can boot linux
> guest on it and inspect SMBIOS tables comfortably.
> 
> 
> With KVM enabled it bails out with:
>    qemu-system-x86_64: kvm_set_user_memory_region: KVM_SET_USER_MEMORY_REGION failed, slot=1, start=0x100000000, size=0x8ff40000000: Invalid argument
> 
> all of that on a host with 32G of RAM/no swap.
> 

#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)

~8 TiB (7,999999)



In QEMU, we have

static hwaddr kvm_max_slot_size = ~0;

And only s390x sets

kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES);

with

#define KVM_SLOT_MAX_BYTES (4UL * TiB)

-- 
Thanks,

David / dhildenb