After discussions with Anthony Liguori, it seems that the virtio balloon can be made even simpler. Here's my attempt. Since the balloon requires Guest cooperation anyway, there seems little reason to force it to tell the Host when it wants to reuse a page. It can simply fault it in. Moreover, the target is best expressed in balloon size, since there is no portable way of getting the total RAM in the system. The host can do the math. Tested with a (fairly hacky) lguest patch. Signed-off-by: Rusty Russell --- drivers/virtio/Kconfig | 10 + drivers/virtio/Makefile | 1 drivers/virtio/virtio_balloon.c | 284 ++++++++++++++++++++++++++++++++++++++++ include/linux/virtio_balloon.h | 18 ++ 4 files changed, 313 insertions(+) diff -r e977a88ffefd drivers/virtio/Kconfig --- a/drivers/virtio/Kconfig Thu Jan 24 12:34:05 2008 +1100 +++ b/drivers/virtio/Kconfig Thu Jan 24 12:40:07 2008 +1100 @@ -23,3 +23,13 @@ config VIRTIO_PCI If unsure, say M. +config VIRTIO_BALLOON + tristate "Virtio balloon driver (EXPERIMENTAL)" + select VIRTIO + select VIRTIO_RING + ---help--- + This driver supports increasing and decreasing the amount + of memory within a KVM guest. + + If unsure, say M. + diff -r e977a88ffefd drivers/virtio/Makefile --- a/drivers/virtio/Makefile Thu Jan 24 12:34:05 2008 +1100 +++ b/drivers/virtio/Makefile Thu Jan 24 12:40:07 2008 +1100 @@ -1,3 +1,4 @@ obj-$(CONFIG_VIRTIO) += virtio.o obj-$(CONFIG_VIRTIO) += virtio.o obj-$(CONFIG_VIRTIO_RING) += virtio_ring.o obj-$(CONFIG_VIRTIO_PCI) += virtio_pci.o +obj-$(CONFIG_VIRTIO_BALLOON) += virtio_balloon.o diff -r e977a88ffefd drivers/virtio/virtio_balloon.c --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/drivers/virtio/virtio_balloon.c Thu Jan 24 12:40:07 2008 +1100 @@ -0,0 +1,284 @@ +/* Virtio balloon implementation, inspired by Dor Loar and Marcelo + * Tosatti's implementations. + * + * Copyright 2008 Rusty Russell IBM Corporation + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, write to the Free Software + * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA + */ +#define DEBUG +#include +#include +#include +#include +#include + +struct virtio_balloon +{ + struct virtio_device *vdev; + struct virtqueue *inflate_vq, *deflate_vq; + + /* Where the ballooning thread waits for config to change. */ + wait_queue_head_t config_change; + + /* The thread servicing the balloon. */ + struct task_struct *thread; + + /* Waiting for host to ack the pages we released. */ + struct completion acked; + + /* Do we have to tell Host *before* we reuse pages? */ + bool tell_host_first; + + /* The pages we've told the Host we're not using. */ + unsigned int num_pages; + struct list_head pages; + + /* The array of pfns we tell the Host about. */ + unsigned int num_pfns; + u32 pfns[256]; +}; + +static struct virtio_device_id id_table[] = { + { VIRTIO_ID_BALLOON, VIRTIO_DEV_ANY_ID }, + { 0 }, +}; + +static void balloon_ack(struct virtqueue *vq) +{ + struct virtio_balloon *vb; + unsigned int len; + + vb = vq->vq_ops->get_buf(vq, &len); + if (vb) + complete(&vb->acked); +} + +static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq) +{ + struct scatterlist sg; + + sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns); + + init_completion(&vb->acked); + + /* We should always be able to add one buffer to an empty queue. */ + if (vq->vq_ops->add_buf(vq, &sg, 1, 0, vb) != 0) + BUG(); + vq->vq_ops->kick(vq); + + /* When host has read buffer, this completes via balloon_ack */ + wait_for_completion(&vb->acked); +} + +static void fill_balloon(struct virtio_balloon *vb, unsigned int num) +{ + /* We can only do one array worth at a time. */ + num = min(num, ARRAY_SIZE(vb->pfns)); + + for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) { + struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY); + if (!page) { + if (printk_ratelimit()) + dev_printk(KERN_INFO, &vb->vdev->dev, + "Out of puff! Can't get %u pages\n", + num); + /* Sleep for at least 1/5 of a second before retry. */ + msleep(200); + break; + } + vb->pfns[vb->num_pfns] = page_to_pfn(page); + totalram_pages--; + vb->num_pages++; + list_add(&page->lru, &vb->pages); + } + + /* Didn't get any? Oh well. */ + if (vb->num_pfns == 0) + return; + + tell_host(vb, vb->inflate_vq); +} + +static void release_pages_by_pfn(const u32 pfns[], unsigned int num) +{ + unsigned int i; + + for (i = 0; i < num; i++) { + __free_page(pfn_to_page(pfns[i])); + totalram_pages++; + } +} + +static void leak_balloon(struct virtio_balloon *vb, unsigned int num) +{ + struct page *page; + + /* We can only do one array worth at a time. */ + num = min(num, ARRAY_SIZE(vb->pfns)); + + for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) { + page = list_first_entry(&vb->pages, struct page, lru); + list_del(&page->lru); + vb->pfns[vb->num_pfns] = page_to_pfn(page); + vb->num_pages--; + } + + if (vb->tell_host_first) { + tell_host(vb, vb->deflate_vq); + release_pages_by_pfn(vb->pfns, vb->num_pfns); + } else { + release_pages_by_pfn(vb->pfns, vb->num_pfns); + tell_host(vb, vb->deflate_vq); + } +} + +static void virtballoon_changed(struct virtio_device *vdev) +{ + struct virtio_balloon *vb = vdev->priv; + + wake_up(&vb->config_change); +} + +static inline int towards_target(struct virtio_balloon *vb) +{ + u32 v; + __virtio_config_val(vb->vdev, + offsetof(struct virtio_balloon_config, num_pages), + &v); + return v - vb->num_pages; +} + +static void update_balloon_size(struct virtio_balloon *vb) +{ + __le32 actual = cpu_to_le32(vb->num_pages); + + vb->vdev->config->set(vb->vdev, + offsetof(struct virtio_balloon_config, actual), + &actual, sizeof(actual)); +} + +static int balloon(void *_vballoon) +{ + struct virtio_balloon *vb = _vballoon; + + set_freezable(); + while (!kthread_should_stop()) { + int diff; + + try_to_freeze(); + wait_event_interruptible(vb->config_change, + (diff = towards_target(vb)) != 0 + || kthread_should_stop()); + if (diff > 0) + fill_balloon(vb, diff); + else if (diff < 0) + leak_balloon(vb, -diff); + update_balloon_size(vb); + } + return 0; +} + +static int virtballoon_probe(struct virtio_device *vdev) +{ + struct virtio_balloon *vb; + int err; + + vdev->priv = vb = kmalloc(sizeof(*vb), GFP_KERNEL); + if (!vb) { + err = -ENOMEM; + goto out; + } + + INIT_LIST_HEAD(&vb->pages); + vb->num_pages = 0; + init_waitqueue_head(&vb->config_change); + vb->vdev = vdev; + + /* We expect two virtqueues. */ + vb->inflate_vq = vdev->config->find_vq(vdev, 0, balloon_ack); + if (IS_ERR(vb->inflate_vq)) { + err = PTR_ERR(vb->inflate_vq); + goto out_free_vb; + } + + vb->deflate_vq = vdev->config->find_vq(vdev, 1, balloon_ack); + if (IS_ERR(vb->deflate_vq)) { + err = PTR_ERR(vb->deflate_vq); + goto out_del_inflate_vq; + } + + vb->thread = kthread_run(balloon, vb, "vballoon"); + if (IS_ERR(vb->thread)) { + err = PTR_ERR(vb->thread); + goto out_del_deflate_vq; + } + + vb->tell_host_first + = vdev->config->feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST); + + return 0; + +out_del_deflate_vq: + vdev->config->del_vq(vb->deflate_vq); +out_del_inflate_vq: + vdev->config->del_vq(vb->inflate_vq); +out_free_vb: + kfree(vb); +out: + return err; +} + +static void virtballoon_remove(struct virtio_device *vdev) +{ + struct virtio_balloon *vb = vdev->priv; + + kthread_stop(vb->thread); + + /* There might be pages left in the balloon: free them. */ + while (vb->num_pages) + leak_balloon(vb, vb->num_pages); + + /* Now we reset the device so we can clean up the queues. */ + vdev->config->reset(vdev); + + vdev->config->del_vq(vb->deflate_vq); + vdev->config->del_vq(vb->inflate_vq); + kfree(vb); +} + +static struct virtio_driver virtio_balloon = { + .driver.name = KBUILD_MODNAME, + .driver.owner = THIS_MODULE, + .id_table = id_table, + .probe = virtballoon_probe, + .remove = __devexit_p(virtballoon_remove), + .config_changed = virtballoon_changed, +}; + +static int __init init(void) +{ + return register_virtio_driver(&virtio_balloon); +} + +static void __exit fini(void) +{ + unregister_virtio_driver(&virtio_balloon); +} +module_init(init); +module_exit(fini); + +MODULE_DEVICE_TABLE(virtio, id_table); +MODULE_DESCRIPTION("Virtio balloon driver"); +MODULE_LICENSE("GPL"); diff -r e977a88ffefd include/linux/virtio_balloon.h --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/include/linux/virtio_balloon.h Thu Jan 24 12:40:07 2008 +1100 @@ -0,0 +1,18 @@ +#ifndef _LINUX_VIRTIO_BALLOON_H +#define _LINUX_VIRTIO_BALLOON_H +#include + +/* The ID for virtio_balloon */ +#define VIRTIO_ID_BALLOON 5 + +/* The feature bitmap for virtio balloon */ +#define VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */ + +struct virtio_balloon_config +{ + /* Number of pages host wants Guest to give up. */ + __le32 num_pages; + /* Number of pages we've actually got in balloon. */ + __le32 actual; +}; +#endif /* _LINUX_VIRTIO_BALLOON_H */