Re: [Qemu-devel] [PATCH v4 08/11] kvm: Persistent per kvmslot dirty bitmap

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> When synchronizing dirty bitmap from kernel KVM we do it in a
> per-kvmslot fashion and we allocate the userspace bitmap for each of
> the ioctl.  This patch instead make the bitmap cache be persistent
> then we don't need to g_malloc0() every time.
> 
> More importantly, the cached per-kvmslot dirty bitmap will be further
> used when we want to add support for the KVM_CLEAR_DIRTY_LOG and this
> cached bitmap will be used to guarantee we won't clear any unknown
> dirty bits otherwise that can be a severe data loss issue for
> migration code.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

OK, so this will use a little more memory, but probably fragment stuff
less.


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  accel/kvm/kvm-all.c      | 10 +++++++---
>  include/sysemu/kvm_int.h |  2 ++
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index b686531586..9fb0694772 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -511,17 +511,19 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml,
>           */
>          size = ALIGN(((mem->memory_size) >> TARGET_PAGE_BITS),
>                       /*HOST_LONG_BITS*/ 64) / 8;
> -        d.dirty_bitmap = g_malloc0(size);
> +        if (!mem->dirty_bmap) {
> +            /* Allocate on the first log_sync, once and for all */
> +            mem->dirty_bmap = g_malloc0(size);
> +        }
>  
> +        d.dirty_bitmap = mem->dirty_bmap;
>          d.slot = mem->slot | (kml->as_id << 16);
>          if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
>              DPRINTF("ioctl failed %d\n", errno);
> -            g_free(d.dirty_bitmap);
>              return -1;
>          }
>  
>          kvm_get_dirty_pages_log_range(section, d.dirty_bitmap);
> -        g_free(d.dirty_bitmap);
>      }
>  
>      return 0;
> @@ -796,6 +798,8 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
>          }
>  
>          /* unregister the slot */
> +        g_free(mem->dirty_bmap);
> +        mem->dirty_bmap = NULL;
>          mem->memory_size = 0;
>          mem->flags = 0;
>          err = kvm_set_user_memory_region(kml, mem, false);
> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
> index f838412491..687a2ee423 100644
> --- a/include/sysemu/kvm_int.h
> +++ b/include/sysemu/kvm_int.h
> @@ -21,6 +21,8 @@ typedef struct KVMSlot
>      int slot;
>      int flags;
>      int old_flags;
> +    /* Dirty bitmap cache for the slot */
> +    unsigned long *dirty_bmap;
>  } KVMSlot;
>  
>  typedef struct KVMMemoryListener {
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK