Re: [Qemu-devel] [PATCH 08/15] virtio: decrease size of VirtQueueElement

[Ming Lei]

On Wed, Jul 30, 2014 at 9:51 PM, Paolo Bonzini <address@hidden> wrote:
> Il 30/07/2014 13:39, Ming Lei ha scritto:
>> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
>> index a60104c..943e72f 100644
>> --- a/include/hw/virtio/virtio.h
>> +++ b/include/hw/virtio/virtio.h
>> @@ -84,12 +84,17 @@ typedef struct VirtQueue VirtQueue;
>>  typedef struct VirtQueueElement
>>  {
>>      unsigned int index;
>> +    unsigned int num;
>>      unsigned int out_num;
>>      unsigned int in_num;
>> -    hwaddr in_addr[VIRTQUEUE_MAX_SIZE];
>> -    hwaddr out_addr[VIRTQUEUE_MAX_SIZE];
>> -    struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
>> -    struct iovec out_sg[VIRTQUEUE_MAX_SIZE];
>> +
>> +    hwaddr *in_addr;
>> +    hwaddr *out_addr;
>> +    struct iovec *in_sg;
>> +    struct iovec *out_sg;
>> +
>> +    hwaddr addr[VIRTQUEUE_MAX_SIZE];
>> +    struct iovec sg[VIRTQUEUE_MAX_SIZE];
>>  } VirtQueueElement;
>>
>>  #define VIRTIO_PCI_QUEUE_MAX 64
>> --
>
> since addr and sg aren't used directly, allocate them flexibly like
>
>     char *p;
>     VirtQueueElement *elem;
>     total_size = ROUND_UP(sizeof(struct VirtQueueElement),
>                           __alignof__(elem->addr[0]);
>     addr_offset = total_size;
>     total_size = ROUND_UP(total_size + num * sizeof(elem->addr[0]),
>                           __alignof__(elem->sg[0]));
>     sg_offset = total_size;
>     total_size += num * sizeof(elem->sg[0]);
>
>     elem = p = g_slice_alloc(total_size);
>     elem->size = total_size;
>     elem->in_addr = p + addr_offset;
>     elem->out_addr = elem->in_addr + in_num;
>     elem->in_sg = p + sg_offset;
>     elem->out_sg = elem->in_sg + in_num;
>
> ...
>
>     g_slice_free1(elem->size, elem);
>
> The small size will help glib do slab-style allocation and should remove
> the need for an object pool.

Yes, that should be correct way to do, but can't avoid big chunk allocation
completely because 'num' is a bit big.

Also this kind of change requires almost all users of elem to be changed,
that need lots of work.

That is why I choose to take the simple approach to ease memory
preallocation for obj pool.

Thanks,