Re: [Qemu-devel] [PATCH/RFC] s390x: split memory into 4TB chunks

[Christian Borntraeger]

On 12/06/2017 01:04 PM, David Hildenbrand wrote:
> On 06.12.2017 13:00, Christian Borntraeger wrote:
>> KVM does not allow memory regions > KVM_MEM_MAX_NR_PAGES, basically
>> limiting the memory per slot to 7.999TB. Lets split the base memory
>> into 4TB chunks to allow go beyond 8TB for a guest. With that (and
>> optimistic overcommitment in the kernel) I was able to start  a 59TB
>> guest on a 1TB system.
>>
>> Signed-off-by: Christian Borntraeger <address@hidden>
>> ---
>>  hw/s390x/s390-virtio-ccw.c | 20 +++++++++++++++++---
>>  1 file changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
>> index 8425534..6735bbb 100644
>> --- a/hw/s390x/s390-virtio-ccw.c
>> +++ b/hw/s390x/s390-virtio-ccw.c
>> @@ -157,11 +157,25 @@ static void virtio_ccw_register_hcalls(void)
>>  static void s390_memory_init(ram_addr_t mem_size)
>>  {
>>      MemoryRegion *sysmem = get_system_memory();
>> -    MemoryRegion *ram = g_new(MemoryRegion, 1);
>> +    ram_addr_t chunk, offset;
>> +    gchar *name;
>>  
>>      /* allocate RAM for core */
>> -    memory_region_allocate_system_memory(ram, NULL, "s390.ram", mem_size);
>> -    memory_region_add_subregion(sysmem, 0, ram);
>> +    offset = 0;
>> +    while (mem_size) {
>> +        MemoryRegion *ram = g_new(MemoryRegion, 1);
>> +        chunk = mem_size;
>> +        /* KVM does not allow memslots >= 8 TB. Lets split into 4TB chunks 
>> */> +        if (chunk > 4UL * 1024 * 1024 * 1024 * 1024) {
>> +            chunk = 4UL * 1024 * 1024 * 1024 * 1024;
>> +        }
>> +        name = g_strdup_printf("s390.ram[0x%lx]", offset);
>> +        memory_region_allocate_system_memory(ram, NULL, name, chunk);
>> +        memory_region_add_subregion(sysmem, offset, ram);
>> +        mem_size -= chunk;
>> +        offset += chunk;
>> +        g_free(name);
>> +    }
>>  
>>      /* Initialize storage key device */
>>      s390_skeys_init();
>>
> 
> This will most certainly break migration, no?

Probably yes. Thats why the patch has RFC. I was looking for ideas.
> 
> 1. I remember the name being used for migration, I might be wrong.
> 2. Migration of guests > 4TB is certainly broken ;)
> 
> I wonder if this should rather be handled in the kvm_slot code.
> (silently create an manage two slots)