Re: [PATCH v19 QEMU 3/4] virtio-balloon: Provide an interface for free page reporting

[David Hildenbrand <david@redhat.com>]

On 15.04.20 10:17, David Hildenbrand wrote:
> On 10.04.20 05:41, Alexander Duyck wrote:
>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>
>> Add support for free page reporting. 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 provides a new way of letting the guest proactively report free
>> pages to the hypervisor, so the hypervisor can reuse them. In contrast to
>> inflate/deflate that is triggered via the hypervisor explicitly.
> 
> Much better, thanks!
> 
>>
>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>> ---
>>  hw/virtio/virtio-balloon.c         |   63 +++++++++++++++++++++++++++++++++++-
>>  include/hw/virtio/virtio-balloon.h |    2 +
>>  2 files changed, 62 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
>> index 1c6d36a29a04..86d8b48a8e3a 100644
>> --- a/hw/virtio/virtio-balloon.c
>> +++ b/hw/virtio/virtio-balloon.c
>> @@ -321,6 +321,57 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>>      balloon_stats_change_timer(s, 0);
>>  }
>>  
>> +static void virtio_balloon_handle_report(VirtIODevice *vdev, VirtQueue *vq)
>> +{
>> +    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
>> +    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() || dev->poison_val) {
>> +                continue;
> 
> actually, you want to do that in the outer loop, no?
> 
>> +            }
>> +
>> +            /*
>> +             * There is no need to check the memory section to see if
>> +             * it is ram/readonly/romd like there is for handle_output
>> +             * below. If the region is not meant to be written to then
>> +             * address_space_map will have allocated a bounce buffer
>> +             * and it will be freed in address_space_unmap and trigger
>> +             * and unassigned_mem_write before failing to copy over the
>> +             * buffer. If more than one bad descriptor is provided it
>> +             * will return NULL after the first bounce buffer and fail
>> +             * to map any resources.
>> +             */
>> +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
>> +            if (!rb) {
>> +                trace_virtio_balloon_bad_addr(elem->in_addr[i]);
>> +                continue;
>> +            }
>> +
>> +            /* For now we will simply ignore unaligned memory regions */
>> +            rb_page_size = qemu_ram_pagesize(rb);
>> +            if (!QEMU_IS_ALIGNED(ram_offset | size, rb_page_size)) {
> 
> /me thinks you can drop rb_page_size
> 
> I *think* there is still one remaining case to handle: Crossing RAM blocks.
> 
> Most probably you should check
> 
> /* For now, ignore crossing RAM blocks. */
> if (ram_offset + size >= qemu_ram_get_used_length()) {
> 	continue;

(should be an > I guess)


-- 
Thanks,

David / dhildenb