On 11/25/2013 10:08 PM, Marcelo Tosatti wrote: > On Mon, Nov 25, 2013 at 02:11:31PM +0800, Xiao Guangrong wrote: >> >> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> >>> >>> 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? > We will check is_last_spte(), all the sptes on parent_ptes should be failed. And Gleb suggested to use a separate slab for rmap, that should be excellent. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
