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Re: [Qemu-devel] [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_P

From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ
Date: Mon, 20 Nov 2017 19:42:40 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 11/14/2017 01:32 AM, Michael S. Tsirkin wrote:
You should Cc Nitesh who is working on a related feature.

OK, I'll do. We have two more issues which haven't been discussed yet, please have a check below.

On Mon, Nov 13, 2017 at 06:34:48PM +0800, Wei Wang wrote:
Ping for comments, thanks.

On 11/03/2017 04:13 PM, Wei Wang wrote:
+static void virtballoon_cmd_report_free_page_start(struct virtio_balloon *vb)
+       unsigned long flags;
+       vb->report_free_page_stop = false;
this flag is used a lot outside any locks. Why is this safe?
Please add some comments explaining access to this flag.

I will revert the logic as suggested: vb->report_free_page. Also plan to simplify its usage as below.

The flag is set or cleared in the config handler according to the new_cmd_id given
by the host:

new_cmd_id=0: WRITE_ONCE(vb->report_free_page, false); // stop reporting new_cmd_id != old_cmd_id: WRITE_ONCE(vb->report_free_page, true); // start reporting

The flag is read by virtio_balloon_send_free_pages() - the callback to report free pages:

if (!READ_ONCE(vb->report_free_page))
                return false;

I don't find where it could be unsafe then (the flag is written by the config handler only).

   static inline s64 towards_target(struct virtio_balloon *vb)
        s64 target;
@@ -597,42 +673,147 @@ static void update_balloon_size_func(struct work_struct 
                queue_work(system_freezable_wq, work);
-static int init_vqs(struct virtio_balloon *vb)
+static bool virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
+                                          unsigned long nr_pages)
-       struct virtqueue *vqs[3];
-       vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request 
-       static const char * const names[] = { "inflate", "deflate", "stats" };
-       int err, nvqs;
+       struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
+       void *addr = (void *)pfn_to_kaddr(pfn);
How do we know all free pages have a kaddr?

For x86_64, it works well since the kernel has all the physical memory mapped already. But for 32-bit kernel, yes, the high memory usually isn't mapped and thus no kaddr. Essentially, this pfn_to_kaddr convert isn't necessary, we do it here because the current API that virtio has is based on "struct scatterlist", which takes a kaddr, and this kaddr is then convert back to physical address in virtqueue_add() when assigning to desc->addr.

I think a better solution would be to add a new API, which directly assigns the caller's guest physical address to desc->addr, similar to the previous implementation "add_one_chunk()" (https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg02452.html). But we can change that to a general virtio API: virtqueue_add_one_desc(struct virtqueue *_vq, u64 base_addr, u32 size, bool in_desc, void *data);

What do you think?


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