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

Marcelo Tosatti
2013-11-25T14:24:49Z

On Mon, Nov 25, 2013 at 02:11:31PM +0800, Xiao Guangrong wrote:
>
> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> > On Wed, Oct 23, 2013 at 09:29:25PM +0800, Xiao Guangrong wrote:
> >> It likes nulls list and we use the pte-list as the nulls which can help us to
> >> detect whether the "desc" is moved to anther rmap then we can re-walk the rmap
> >> if that happened
> >>
> >> kvm->slots_lock is held when we do lockless walking that prevents rmap
> >> is reused (free rmap need to hold that lock) so that we can not see the same
> >> nulls used on different rmaps
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> >
> > How about simplified lockless walk on the slot while rmapp entry
> > contains a single spte? (which should be the case with two-dimensional
> > paging).
> >
> > That is, grab the lock when finding a rmap with more than one spte in
> > it (and then keep it locked until the end).
>
> Hmm… that isn't straightforward and more complex than the approach
> in this patchset. Also it can drop the improvement for shadow mmu that
> gets great improvement by this patchset.

It is not more complex, since it would remove list lockless walk. Only
the spte pointer at rmap[spte] is accessed without a lock. Its much much
simpler.

> > For example, nothing prevents lockless walker to move into some
> > parent_ptes chain, right?
>
> No.
>
> The nulls can help us to detect this case, for parent_ptes, the nulls points
> to "shadow page" but for rmaps, the nulls points to slot.arch.rmap. There
> is no chance that the “rmap" is used as shadow page when slot-lock is held.

The SLAB cache is the same, so entries can be reused. What prevents
a desc entry living in slot.arch.rmap to be freed and reused by a
parent_ptes desc?

> > 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?
>
> But… do we really need to care it. :(

See my reply to Avi.