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Date: Wed, 30 Jul 2014 15:51:22 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 08/15] virtio: decrease size of
	VirtQueueElement
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To: Ming Lei <ming.lei@canonical.com>, qemu-devel@nongnu.org, Peter Maydell <peter.maydell@linaro.org>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

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.

Paolo