Re: [Qemu-devel] [PATCH/RFC v2] s390x: start a new memory region if the old one exceeds KVM_MEM_MAX_NR_PAGES

[David Hildenbrand]

[resending as I noticed that I dropped the ccs when I sent this last week]

On 07.12.2017 15:58, Christian Borntraeger wrote:
> KVM does not allow memory regions > KVM_MEM_MAX_NR_PAGES, basically
> limiting the memory per slot to 8TB-4k. Lets start a new memory region
> if we cross that boundary.
>
> 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>
> ---

This should not harm migration I guess. And it's easier than the other
alternatives we discussed.

>  hw/s390x/s390-virtio-ccw.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> index 8425534..3630f6a 100644
> --- a/hw/s390x/s390-virtio-ccw.c
> +++ b/hw/s390x/s390-virtio-ccw.c
> @@ -154,14 +154,41 @@ 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.
> + */
> +#define KVM_SLOT_MAX ((((1UL << 31) - 1) * 4096) & ~0xfffffUL )

KVM_SLOT_MAX_BYTES ?

(4096 -> TARGET_PAGE_SIZE, 0xfffff -> ? )

> +
>  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;
> +    unsigned int number;
> +    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;
> +    number = 0;

you can initialize these directly

> +    name = g_strdup_printf("s390.ram");
> +    while (mem_size) {
> +        MemoryRegion *ram = g_new(MemoryRegion, 1);
> +        chunk = mem_size;
> +        /* KVM does not allow memslots >= 8 TB */
> +        if (chunk > KVM_SLOT_MAX) {
> +            chunk = KVM_SLOT_MAX;
> +        }

chunk = MIN() ...

> +        memory_region_allocate_system_memory(ram, NULL, name, chunk);
> +        memory_region_add_subregion(sysmem, offset, ram);
> +        mem_size -= chunk;
> +        offset += chunk;
> +     number++;
> +        g_free(name);
> +        name = g_strdup_printf("s390.ram.%u", number);

you could directly use number++ here, when initializing number to 1.
(then also the strange indentation of number++ above is gone  )

> +    }
> +    g_free(name);
>
>      /* Initialize storage key device */
>      s390_skeys_init();
>


-- 

Thanks,

David / dhildenb