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

[David Hildenbrand <david@redhat.com>]

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 <borntraeger@de.ibm.com>
> ---
>  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

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 <david@redhat.com>

> +        /* 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();
> 


-- 

Thanks,

David / dhildenb