Re: [PATCH] KVM: Remove duplicated zero clear with dirty_bitmap buffer

[maobibo]

On 2024/6/14 上午3:25, Christophe JAILLET wrote:
> Le 13/06/2024 à 14:54, Bibo Mao a écrit :
> > Since dirty_bitmap pointer is allocated with function __vcalloc(),
> > there is __GFP_ZERO flag set in the implementation about this function
> > __vcalloc_noprof(). It is not necessary to clear dirty_bitmap buffer
> > with zero again.
> >
> > Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> > ---
> >   virt/kvm/kvm_main.c | 3 ---
> >   1 file changed, 3 deletions(-)
>
> Hi,
>
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 14841acb8b95..c7d4a041dcfa 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1669,9 +1669,6 @@ static int kvm_prepare_memory_region(struct kvm 
> > *kvm,
> >               r = kvm_alloc_dirty_bitmap(new);
> >               if (r)
> >                   return r;
> > -
> > -            if (kvm_dirty_log_manual_protect_and_init_set(kvm))
> > -                bitmap_set(new->dirty_bitmap, 0, new->npages);
>
> unless I miss something obvious, this does not clear anything, but set 
> all bits to 1.
>
> 0 is not for "write 0" (i.e. clear), but for "start at offset 0".
> So all bits are set to 1 in this case.
you are right, I had thought it is to write 0 :(

I do not know whether KVM_DIRTY_LOG_INITIALLY_SET should be enabled on 
LoongArch. If it is set, write protection for second MMU will start one 
by one in function kvm_arch_mmu_enable_log_dirty_pt_masked() when dirty 
log is cleared if it is set, else write protection will start in 
function kvm_arch_commit_memory_region() when flag of memslot is changed.

I do not see the obvious benefits between these two write protect 
stages. Can anyone give me any hints?

Regards
Bibo Mao

> CJ
>
> >           }
> >       }
> >
> > base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670