Re: [PATCH RFCv1 4/8] kvm: Introduce secondary dirty bitmap

[Gavin Shan]

On 2/9/23 9:07 AM, Juan Quintela wrote:
> Gavin Shan <gshan@redhat.com> wrote:
> > When dirty ring is enabled on ARM64, the backup bitmap may be used
> > to track the dirty pages in no-running-vcpu situations. The original
> > bitmap is the primary one, used for the dirty ring buffer. We need
> > the secondary bitmap to collect the backup bitmap for ARM64.
> >
> > No functional change intended.
> >
> > Signed-off-by: Gavin Shan <gshan@redhat.com>
> > ---
> >   accel/kvm/kvm-all.c      | 50 ++++++++++++++++++++++++++++++----------
> >   include/sysemu/kvm_int.h |  1 +
> >   2 files changed, 39 insertions(+), 12 deletions(-)
> >
> > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> > index 01a6a026af..1a93985574 100644
> > --- a/accel/kvm/kvm-all.c
> > +++ b/accel/kvm/kvm-all.c
> > @@ -553,13 +553,29 @@ static void kvm_log_stop(MemoryListener *listener,
> >       }
> >   }
> >   
> > +static unsigned long *kvm_slot_dirty_bitmap(KVMSlot *slot, bool primary)
> > +{
> > +    if (primary) {
> > +        return slot->dirty_bmap;
> > +    }
> > +
> > +    return slot->dirty_bmap +
> > +           slot->dirty_bmap_size / sizeof(slot->dirty_bmap[0]);
> > +}
>
>
> Why?
> Just use two bitmaps and call it a day.

Thanks for your review, Juan. Right, I had wrong assumption that the original
(primary) bitmap can't be reused. It's why the secondary bitmap is introduced.
The intention is to use the original (primary) bitmap to cover the dirty-ring
buffers while the secondary bitmap is to cover the backup bitmap, resident in
host kernel.

I think the original (primary) bitmap can be reused in this case. After the
dirty-ring buffer is synchronized to the original bitmap, which is updated
to the dirty bits. It can be reused to cover the backup bitmap. I will remove
the secondary bitmap in next revision.

Thanks,
Gavin