[Qemu-devel] Re: [COMMIT bd83677] Fake dirty loggin when it's not there

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> From: Alexander Graf <agraf-l3A5Bk7waGM@public.gmane.org>
> 
> Some KVM platforms don't support dirty logging yet, like IA64 and PPC,
> so in order to still have screen updates on those, we need to fake it.
> 
> This patch just tells the getter function for dirty bitmaps, that all
> pages within a slot are dirty when the slot has dirty logging enabled.
> 
> That way we can implement dirty logging on those platforms sometime when
> it drags down performance, but share the rest of the code with dirty
> logging capable platforms.

Sorry to disturb the merge, but my remarks remained unpatched. Would be
nice if they are considered in some follow-up patch... :)

> 
> Signed-off-by: Alexander Graf <agraf-l3A5Bk7waGM@public.gmane.org>
> Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
> 
> diff --git a/kvm-all.c b/kvm-all.c
> index 961fa32..2032949 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -300,6 +300,7 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>      KVMDirtyLog d;
>      KVMSlot *mem;
>      int ret = 0;
> +    int r;
>  
>      d.dirty_bitmap = NULL;
>      while (start_addr < end_addr) {
> @@ -308,6 +309,11 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>              break;
>          }
>  
> +        /* We didn't activate dirty logging? Don't care then. */
> +        if(!(mem->flags & KVM_MEM_LOG_DIRTY_PAGES)) {
> +            continue;
> +        }
> +

According to Alex' reply [1], I think this is probably better expressed
as an assert() than a silent skip. It indicates an error at higher
level, no?

>          size = ((mem->memory_size >> TARGET_PAGE_BITS) + 7) / 8;
>          if (!d.dirty_bitmap) {
>              d.dirty_bitmap = qemu_malloc(size);
> @@ -319,7 +325,8 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>  
>          d.slot = mem->slot;
>  
> -        if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
> +        r = kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d);
> +        if (r == -EINVAL) {
>              dprintf("ioctl failed %d\n", errno);
>              ret = -1;
>              break;

My remark still stands: This is an "optimistic" assumption that only
EINVAL is a "real" error. If KVM is buggy /wrt a specific invalid return
value, work around that particular bug (and we should also fix the
kernel, but that's a different topic).

Jan

[1] http://permalink.gmane.org/gmane.comp.emulators.qemu/47996