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? > > 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. -- 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
