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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 29 Mar 2016 12:05:57 +0200
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Subject: Re: [Qemu-devel] [PATCH v2] migration: skip sending ram pages
 released by virtio-balloon driver.
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To: "Michael S. Tsirkin" <mst@redhat.com>, Jitendra Kolhe <jitendra.kolhe@hpe.com>
Cc: JBottomley@Odin.com, ehabkost@redhat.com, crosthwaite.peter@gmail.com, simhan@hpe.com, armbru@redhat.com, quintela@redhat.com, qemu-devel@nongnu.org, lcapitulino@redhat.com, borntraeger@de.ibm.com, mohan_parthasarathy@hpe.com, stefanha@redhat.com, amit.shah@redhat.com, den@openvz.org, dgilbert@redhat.com, rth@twiddle.net



On 28/03/2016 08:59, Michael S. Tsirkin wrote:
>> > +    qemu_mutex_lock_balloon_bitmap();
>> >      for (;;) {
>> >          size_t offset = 0;
>> >          uint32_t pfn;
>> >          elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
>> >          if (!elem) {
>> > +            qemu_mutex_unlock_balloon_bitmap();
>> >              return;
>> >          }
>> >  
>> > @@ -242,6 +245,7 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>> >              addr = section.offset_within_region;
>> >              balloon_page(memory_region_get_ram_ptr(section.mr) + addr,
>> >                           !!(vq == s->dvq));
>> > +            qemu_balloon_bitmap_update(addr, !!(vq == s->dvq));
>> >              memory_region_unref(section.mr);
>> >          }
>> >  
> So the assumption here is that offset_within_region equals
> ram ptr if region is get_system_memory.
> 
> And I'm not sure that's always right.
> 
> Paolo?

Indeed.  It is correct for the main system RAM, but hot-plugged RAM
would also have a zero-based section.offset_within_region.  You need to
add memory_region_get_ram_addr(section.mr), just like the call to
balloon_page adds memory_region_get_ram_ptr(section.mr).

Paolo