Re: [Patch v2 0/4] KVM: Guest page hinting

[Wei Wang <wei.w.wang@intel.com>]

On 09/14/2017 12:17 AM, Nitesh Narayan Lal wrote:
> Changelog in v2:
>          - Addressed comments provided in v1
>          - Implementation to pass the global hyperlist (carrying pages which are to be freed) to the host by using existing virtio-balloon infrastructure (deflate_vq).
>
> I am using synchronous virtqueue_kick API for which changes in virtio_ring.c and virtio.h are picked from Wei Wang's patch-set for Virtio-balloon enhancement [2]. ("Wei, How would you like me to credit you in the final patch?"). I am using this API because it doesn't requires any memory allocation in order to pass the list of pfns to the host. Which is required because of the seqlock used in arch_free_page to prevent memory allocations.
>
> Query:
>          - So far I don't have any implementation in QEMU. I have added few prints in QEMU balloon code to ensure that the guest page hinting kick is landing in the right location. As per my understanding on using deflate_vq QEMU's "virtio_balloon_handle_output" in hw/virtio/virtio-balloon.c should be invoked after the kick. But in my case for some reason that is not getting invoked. Any suggestions where am I going wrong?
>
> [1] http://www.spinics.net/lists/kvm/msg153666.html
> [2] http://www.spinics.net/lists/kvm/msg152734.html;
>

Hi Nitesh,

I had a quick look of this approach, and have some high-level questions:

1) What's the usage of the feature, in addition to accelerating live 
migration?

2) Are the free page hints continuously added to and removed from the 
per CPU arrays during the whole lifecycle of the guest whenever 
alloc/free is invoked?

3) The per-CPU arrays are sync-ed to a hypervisor page array under a 
lock. If all the CPUs happen to do the sync at the same time, the later 
ones may be possible to spin too long?


Best,
Wei