On Fri, Apr 12, 2013 at 01:41:58PM +0100, Will Deacon wrote:
> Hi Rusty,
>
> On Fri, Apr 12, 2013 at 12:36:00PM +0100, Rusty Russell wrote:
> > I'm confused. kvmtool is using too much memory, or the guest? If
> > kvmtool, the Device Initialization section above applies to the driver,
> > not the device. If the guest, well, the language says "UP TO N+1". You
> > want a small guest, don't use them all. Or any...
> >
> > What am I missing?
>
> I don't think you're missing anything. The memory usage is in the guest, not
> kvmtool. If, as you suggest, the spec doesn't mandate greedy initialisation
> of all the virtqueues, then it sounds like a bug in Linux.
>
> Looking at virtio_mmio.c:
>
> /* Allocate pages for the queue - start with a queue as big as
> * possible (limited by maximum size allowed by device), drop down
> * to a minimal size, just big enough to fit descriptor table
> * and two rings (which makes it "alignment_size * 2")
> */
> info->num = readl(vm_dev->base + VIRTIO_MMIO_QUEUE_NUM_MAX);
>
> /* If the device reports a 0 entry queue, we won't be able to
> * use it to perform I/O, and vring_new_virtqueue() can't create
> * empty queues anyway, so don't bother to set up the device.
> */
> if (info->num == 0) {
> err = -ENOENT;
> goto error_alloc_pages;
> }
>
> while (1) {
> size = PAGE_ALIGN(vring_size(info->num,
> VIRTIO_MMIO_VRING_ALIGN));
> /* Did the last iter shrink the queue below minimum size? */
> if (size < VIRTIO_MMIO_VRING_ALIGN * 2) {
> err = -ENOMEM;
> goto error_alloc_pages;
> }
>
> info->queue = alloc_pages_exact(size, GFP_KERNEL | __GFP_ZERO);
> if (info->queue)
> break;
>
> info->num /= 2;
> }
>
> so, although there is some back-off if the allocation fails, it doesn't really
> help userspace when there is enough memory for the maximal buffers, but not
> a lot else.
>
> Will
Well, that's just a couple of pages per virtqueue. How many of these do
you have?
--
MST
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