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Subject: Re: [virtio] [OASIS Issue Tracker] Created: (VIRTIO-28) Implement new balloon device (ID 13)


Sorry for late reply but I am very busy now.

On Tue, Oct 01, 2013 at 05:40:12PM +0930, Rusty Russell wrote:
> Ok, so here's what I ended up with.
> Any feedback welcome...
> Rusty.


> + Memory Balloon Device
> +===========================
> +
> +The virtio memory balloon device is a primitive device for managing
> +guest memory: the device asks for a certain amount of memory, and the
> +guest supplies it.  This allows the guest to adapt to changes in
> +allowance of underlying physical memory.  The device can also be used
> +to communicate guest memory statistics to the host.

Maybe guest should also be able to set balloon target. However, in this
situation host should establish limits which could not be exceeded and
device should enforce them. This way balloon could be controlled from host
and/or guest if needed. So balloon device will be responsible just for
passing requests to/from guest/host and limits enforcement. This way even
memory hotplug could be easily implemented. However, in this situation
device should not be called balloon. Memory manager or memory allocator?
Any better ideas?

> + Device ID
> +-----------------
> +  13
> +
> + Virtqueues
> +------------------
> +  0:inputq. 1:outputq.
> +
> +
> + Feature bits
> +--------------------
> +  None defined.
> +
> + Device configuration layout
> +-----------------------------------
> +
> +The read-only configuration field indicates the granularity of memory
> +which can be added to the balloon.  This is typically reflects the
> +page size of the host (eg. 12 for 4096-byte pages).
> +
> +	struct virtio_balloon_config {
> +		u32 page_bits;
> +	}

Why balloon device must be forced to use only one page size? I think that
configuration area should list which page sizes could be requested by
device. Device should be able to request any allowed/defined size but driver
could reject request or fail partially. Additionally, maybe device should
inform about allowed page sizes using size explicitly instead of number
of bits. So maybe it is worth storing page sizes as u64. By the way we could
store page sizes which are not equal full power of 2 (could be useful for
some strange sizes of superpages in the future if something crazy happens).
However, if we store page sizes as number of bits we could represent
larger sizes. Hmmm...

Probably we do not implement above mentioned feature at first but
it gives us a chance to do that later.

> + Device Initialization
> +-----------------------------
> +
> +1. At least one struct virtio_balloon_request buffer should be placed
> +   in the inputq.
> +
> +2. The balloon starts empty (size 0).
> +
> + Device Operation
> +------------------------
> +
> +The device is driven by receipt of a command in the input queue:
> +
> +	struct virtio_balloon_req {
> +		u32 type;
> +		u32 reserved;
> +		u64 value;
> +	}

struct virtio_balloon_pages {
  u32 type;
  u32 reserved;
  u64 guest_memory_size;
  u64 page_sizes[];
}; ???

> +1. A VIRTIO_BALLOON_REQ_RESIZE command indicates the balloon target
> +   size (in bytes) in the value field.  If the current balloon size is
> +   smaller than the target, the guest should add pages to the balloon
> +   as soon as possible.  If the current balloon is larger than the
> +   target, the guest may withdraw pages.
> +
> +2. To add pages to the balloon, the physical addresses of the pages

frames ???

> +   are sent using the output queue.  The number of pages is implied in
> +   the message length, and each page value must be a multiple of the
> +   page size indicated in struct virtio_balloon_config.
> +
> +	struct virtio_balloon_pages {
> +		u64 page[];
> +	};

struct virtio_balloon_pages {
  u64 page_size;
  u64 frames[];
}; ???

> +3. To withdraw a page from the balloon, it can simply be accessed.

IIRC, ballooned pages are at first reserved and later frames are returned
to a host. So if you would like to use pages you must do above steps
in revers. Hence, "it can simply be accessed" is a bit misleading. May it
should be phrased in following way: it should have reassigned a frame
number and later it should be returned to a pool of free pages.

Additionally, some hypervisors my require additional steps to add/remove
page to/from the pool (e.g. Xen PV guests must add/remove frames to P2M and M2P
lists/trees too). So implementation should be able to call hypervisor
specific stuff in such situations.


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