Re: [PATCH 1/7] KVM: Write protect the updated slot only when we start dirty logging

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jan 07, 2013 at 06:11:46PM -0200, Marcelo Tosatti wrote:
> On Tue, Dec 18, 2012 at 04:26:47PM +0900, Takuya Yoshikawa wrote:
> > This is needed to make kvm_mmu_slot_remove_write_access() rmap based:
> > otherwise we may end up using invalid rmap's.
> > 
> > Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
> 
> Why? memslot->arch.rmap[] has been properly allocated at this point.
> 
FWIW a long time ago in a galaxy far, far away there was a check for
KVM_MEM_LOG_DIRTY_PAGES before call to kvm_mmu_slot_remove_write_access(),
but it was removed by 90cb0529dd230548a7, as far as I can tell, accidentally.

> > ---
> >  arch/x86/kvm/x86.c  |    9 ++++++++-
> >  virt/kvm/kvm_main.c |    1 -
> >  2 files changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 1c9c834..9451efa 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6897,7 +6897,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >  	spin_lock(&kvm->mmu_lock);
> >  	if (nr_mmu_pages)
> >  		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> > -	kvm_mmu_slot_remove_write_access(kvm, mem->slot);
> > +	/*
> > +	 * Write protect all pages for dirty logging.
> > +	 * Existing largepage mappings are destroyed here and new ones will
> > +	 * not be created until the end of the logging.
> > +	 */
> > +	if ((mem->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
> > +	    !(old.flags & KVM_MEM_LOG_DIRTY_PAGES))
> > +		kvm_mmu_slot_remove_write_access(kvm, mem->slot);
> >  	spin_unlock(&kvm->mmu_lock);
> >  	/*
> >  	 * If memory slot is created, or moved, we need to clear all
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index bd31096..0ef5daa 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -805,7 +805,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  	if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
> >  		if (kvm_create_dirty_bitmap(&new) < 0)
> >  			goto out_free;
> > -		/* destroy any largepage mappings for dirty tracking */
> >  	}
> >  
> >  	if (!npages || base_gfn != old.base_gfn) {
> > -- 
> > 1.7.5.4
> > 
> > --
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--
			Gleb.