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Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble
 hinting"
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     kvm@vger.kernel.org, david@redhat.com, dave.hansen@intel.com,
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        akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <ee5387b1-89af-daf4-8492-8139216c6dcf@redhat.com>
Date:   Wed, 24 Jul 2019 16:29:27 -0400
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On 7/24/19 4:18 PM, Alexander Duyck wrote:
> On Wed, 2019-07-24 at 15:02 -0400, Michael S. Tsirkin wrote:
>> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>
>>> Add support for what I am referring to as "bubble hinting". Basically the
>>> idea is to function very similar to how the balloon works in that we
>>> basically end up madvising the page as not being used. However we don't
>>> really need to bother with any deflate type logic since the page will be
>>> faulted back into the guest when it is read or written to.
>>>
>>> This is meant to be a simplification of the existing balloon interface
>>> to use for providing hints to what memory needs to be freed. I am assuming
>>> this is safe to do as the deflate logic does not actually appear to do very
>>> much other than tracking what subpages have been released and which ones
>>> haven't.
>>>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>> ---
>>>  hw/virtio/virtio-balloon.c                      |   40 +++++++++++++++++++++++
>>>  include/hw/virtio/virtio-balloon.h              |    2 +
>>>  include/standard-headers/linux/virtio_balloon.h |    1 +
>>>  3 files changed, 42 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
>>> index 2112874055fb..70c0004c0f88 100644
>>> --- a/hw/virtio/virtio-balloon.c
>>> +++ b/hw/virtio/virtio-balloon.c
>>> @@ -328,6 +328,39 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>>>      balloon_stats_change_timer(s, 0);
>>>  }
>>>  
>>> +static void virtio_bubble_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>>> +{
>>> +    VirtQueueElement *elem;
>>> +
>>> +    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
>>> +    	unsigned int i;
>>> +
>>> +        for (i = 0; i < elem->in_num; i++) {
>>> +            void *addr = elem->in_sg[i].iov_base;
>>> +            size_t size = elem->in_sg[i].iov_len;
>>> +            ram_addr_t ram_offset;
>>> +            size_t rb_page_size;
>>> +            RAMBlock *rb;
>>> +
>>> +            if (qemu_balloon_is_inhibited())
>>> +                continue;
>>> +
>>> +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
>>> +            rb_page_size = qemu_ram_pagesize(rb);
>>> +
>>> +            /* For now we will simply ignore unaligned memory regions */
>>> +            if ((ram_offset | size) & (rb_page_size - 1))
>>> +                continue;
>>> +
>>> +            ram_block_discard_range(rb, ram_offset, size);
>> I suspect this needs to do like the migration type of
>> hinting and get disabled if page poisoning is in effect.
>> Right?
> Shouldn't something like that end up getting handled via
> qemu_balloon_is_inhibited, or did I miss something there? I assumed cases
> like that would end up setting qemu_balloon_is_inhibited to true, if that
> isn't the case then I could add some additional conditions. I would do it
> in about the same spot as the qemu_balloon_is_inhibited check.
I don't think qemu_balloon_is_inhibited() will take care of the page poisoning
situations.
If I am not wrong we may have to look to extend VIRTIO_BALLOON_F_PAGE_POISON
support as per Michael's suggestion.
>
>
-- 
Thanks
Nitesh