Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Nov 26, 2013 at 11:21:37AM +0800, Xiao Guangrong wrote:
> On 11/26/2013 02:12 AM, Marcelo Tosatti wrote:
> > On Mon, Nov 25, 2013 at 02:29:03PM +0800, Xiao Guangrong wrote:
> >>>> Also, there is no guarantee of termination (as long as sptes are
> >>>> deleted with the correct timing). BTW, can't see any guarantee of
> >>>> termination for rculist nulls either (a writer can race with a lockless
> >>>> reader indefinately, restarting the lockless walk every time).
> >>>
> >>> Hmm, that can be avoided by checking dirty-bitmap before rewalk,
> >>> that means, if the dirty-bitmap has been set during lockless write-protection,
> >>> it�s unnecessary to write-protect its sptes. Your idea?
> >> This idea is based on the fact that the number of rmap is limited by
> >> RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap,
> >> we can break the rewalk at once, in the case of deleting, we can only
> >> rewalk RMAP_RECYCLE_THRESHOLD times.
> > 
> > Please explain in more detail.
> 
> Okay.
> 
> My proposal is like this:
> 
> pte_list_walk_lockless()
> {
> restart:
> 
> +	if (__test_bit(slot->arch.dirty_bitmap, gfn-index))
> +		return;
> 
> 	code-doing-lockless-walking;
> 	......
> }
> 
> Before do lockless-walking, we check the dirty-bitmap first, if
> it is set we can simply skip write-protection for the gfn, that
> is the case that new spte is being added into rmap when we lockless
> access the rmap.

The dirty bit could be set after the check.

> For the case of deleting spte from rmap, the number of entry is limited
> by RMAP_RECYCLE_THRESHOLD, that is not endlessly.

It can shrink and grow while lockless walk is performed.