On 11/25/2013 02:11 PM, 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. > >> >> 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. > >> >> 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. -- 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
