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Subject: Re: [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote:
On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote:On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote:On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote:Signed-off-by: Wei Wang <wei.w.wang@intel.com> Signed-off-by: Liang Li <liang.z.li@intel.com> CC: Michael S. Tsirkin <mst@redhat.com> CC: Dr. David Alan Gilbert <dgilbert@redhat.com> CC: Juan Quintela <quintela@redhat.com>I find it suspicious that neither unrealize nor reset functions have been touched at all. Are you sure you have thought through scenarious like hot-unplug or disabling the device by guest?OK. I think we can call balloon_free_page_stop in unrealize and reset.+static void *virtio_balloon_poll_free_page_hints(void *opaque) +{ + VirtQueueElement *elem; + VirtIOBalloon *dev = opaque; + VirtQueue *vq = dev->free_page_vq; + uint32_t id; + size_t size; What makes it safe to poke at this device from multiple threads? I think that it would be safer to do it from e.g. BH.Actually the free_page_optimization thread is the only user of free_page_vq, and there is only one optimization thread each time. Would this be safe enough? Best, WeiAren't there other fields there? Also things like reset affect all VQs.
Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue here.
The potential issue I could observe here is that "dev->free_page_report_status" is read and written by the optimization thread, and it is also modified by the migration thread and reset via virtio_balloon_free_page_stop.
How about adding a QEMU SpinLock, like this: virtio_balloon_poll_free_page_hints() { while (1) { qemu_spin_lock();/* If the status has been changed to STOP or EXIT, or the VM is stopped, just exit */ if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP || !runstate_is_running()) {
qemu_spin_unlock(); break; } .... qemu_spin_unlock(); } } Best, Wei
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