Re: [qemu-s390x] [PATCH v3 1/1] s390-ccw-virtio: allow for systems larger that 7.999TB

[Christian Borntraeger]

On 12/11/2017 02:55 PM, David Hildenbrand wrote:
> On 11.12.2017 13:21, Christian Borntraeger wrote:
>> KVM does not allow memory regions > KVM_MEM_MAX_NR_PAGES, basically
>> limiting the memory per slot to 8TB-4k. As memory slots on s390/kvm must
>> be a multiple of 1MB we need start a new memory region if we cross
>> 8TB-1M.
>>
>> With that (and optimistic overcommitment in the kernel) I was able to
>> start a 24TB guest on a 1TB system.
>>
>> Signed-off-by: Christian Borntraeger <address@hidden>
>> ---
>>  hw/s390x/s390-virtio-ccw.c | 28 +++++++++++++++++++++++++---
>>  1 file changed, 25 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
>> index 8425534..073f6ed 100644
>> --- a/hw/s390x/s390-virtio-ccw.c
>> +++ b/hw/s390x/s390-virtio-ccw.c
>> @@ -154,14 +154,36 @@ static void virtio_ccw_register_hcalls(void)
>>                                     virtio_ccw_hcall_early_printk);
>>  }
>>  
>> +/*
>> + * KVM does only support memory slots up to KVM_MEM_MAX_NR_PAGES pages
>> + * as the dirty bitmap must be managed by bitops that take an int as
>> + * position indicator. If we have a guest beyond that we will split off
>> + * new subregions. The split must happen on a segment boundary (1MB).
>> + */
>> +#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)
>> +#define SEG_MSK (~0xfffffULL)
>> +#define KVM_SLOT_MAX_BYTES ((KVM_MEM_MAX_NR_PAGES * TARGET_PAGE_SIZE) & 
>> SEG_MSK)
> 
> Just wondering if we could get into trouble when calculating
> 
> KVM_MEM_MAX_NR_PAGES * TARGET_PAGE_SIZE

maybe just using 1ULL instead of 1UL?


> 
> on a host with sizeof(long) == 4
> 
> could it wrap? (e.g. crazy mingw stuff)
> 
>>  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 = 0;
>> +    unsigned int number = 0;
>> +    gchar *name;
>>  
>>      /* allocate RAM for core */
>> -    memory_region_allocate_system_memory(ram, NULL, "s390.ram", mem_size);
>> -    memory_region_add_subregion(sysmem, 0, ram);
>> +    name = g_strdup_printf("s390.ram");
>> +    while (mem_size) {
>> +        MemoryRegion *ram = g_new(MemoryRegion, 1);
> 
> I'd add an empty line here.
> 
> Reviewed-by: David Hildenbrand <address@hidden>
> 
>> +        /* KVM does not allow memslots >= 8 TB */
>> +        chunk = MIN(mem_size, KVM_SLOT_MAX_BYTES);
>> +        memory_region_allocate_system_memory(ram, NULL, name, chunk);
>> +        memory_region_add_subregion(sysmem, offset, ram);
>> +        mem_size -= chunk;
>> +        offset += chunk;
>> +        g_free(name);
>> +        name = g_strdup_printf("s390.ram.%u", ++number);
>> +    }
>> +    g_free(name);
>>  
>>      /* Initialize storage key device */
>>      s390_skeys_init();
>>
> 
>