Re: [PATCH v1 3/6] kvm: Synchronize the backup bitmap in the last stage

[Peter Xu <peterx@redhat.com>]

On Mon, Feb 13, 2023 at 08:39:22AM +0800, Gavin Shan wrote:
> In the last stage of live migration or memory slot removal, the
> backup bitmap needs to be synchronized when it has been enabled.
> 
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  accel/kvm/kvm-all.c      | 11 +++++++++++
>  include/sysemu/kvm_int.h |  1 +
>  2 files changed, 12 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 01a6a026af..b5e12de522 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -1352,6 +1352,10 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
>                   */
>                  if (kvm_state->kvm_dirty_ring_size) {
>                      kvm_dirty_ring_reap_locked(kvm_state, NULL);
> +                    if (kvm_state->kvm_dirty_ring_with_bitmap) {
> +                        kvm_slot_sync_dirty_pages(mem);
> +                        kvm_slot_get_dirty_log(kvm_state, mem);
> +                    }
>                  } else {
>                      kvm_slot_get_dirty_log(kvm_state, mem);
>                  }

IIUC after the memory atomic update changes lands QEMU, we may not need
this sync at all.

My understanding is that we sync dirty log here only because of non-atomic
updates happening in the past and we may lose dirty bits unexpectedly.
Maybe Paolo knows.

But that needs some more justification and history digging, so definitely
more suitable to leave it for later and separate discussion.

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks,

> @@ -1573,6 +1577,12 @@ static void kvm_log_sync_global(MemoryListener *l, bool last_stage)
>          mem = &kml->slots[i];
>          if (mem->memory_size && mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
>              kvm_slot_sync_dirty_pages(mem);
> +
> +            if (s->kvm_dirty_ring_with_bitmap && last_stage &&
> +                kvm_slot_get_dirty_log(s, mem)) {
> +                kvm_slot_sync_dirty_pages(mem);
> +            }
> +
>              /*
>               * This is not needed by KVM_GET_DIRTY_LOG because the
>               * ioctl will unconditionally overwrite the whole region.
> @@ -3701,6 +3711,7 @@ static void kvm_accel_instance_init(Object *obj)
>      s->kernel_irqchip_split = ON_OFF_AUTO_AUTO;
>      /* KVM dirty ring is by default off */
>      s->kvm_dirty_ring_size = 0;
> +    s->kvm_dirty_ring_with_bitmap = false;
>      s->notify_vmexit = NOTIFY_VMEXIT_OPTION_RUN;
>      s->notify_window = 0;
>  }
> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
> index 60b520a13e..fdd5b1bde0 100644
> --- a/include/sysemu/kvm_int.h
> +++ b/include/sysemu/kvm_int.h
> @@ -115,6 +115,7 @@ struct KVMState
>      } *as;
>      uint64_t kvm_dirty_ring_bytes;  /* Size of the per-vcpu dirty ring */
>      uint32_t kvm_dirty_ring_size;   /* Number of dirty GFNs per ring */
> +    bool kvm_dirty_ring_with_bitmap;
>      struct KVMDirtyRingReaper reaper;
>      NotifyVmexitOption notify_vmexit;
>      uint32_t notify_window;
> -- 
> 2.23.0
> 

-- 
Peter Xu