On Thu, Aug 25, 2011 at 03:57:22PM +0800, Xiao Guangrong wrote: > On 08/24/2011 03:09 AM, Marcelo Tosatti wrote: > > On Wed, Aug 24, 2011 at 12:32:32AM +0800, Xiao Guangrong wrote: > >> On 08/23/2011 08:38 PM, Marcelo Tosatti wrote: > >> > >>>> And, i think there are not problems since: if the spte without accssed bit is > >>>> written frequently, it means the guest page table is accessed infrequently or > >>>> during the writing, the guest page table is not accessed, in this time, zapping > >>>> this shadow page is not bad. > >>> > >>> Think of the following scenario: > >>> > >>> 1) page fault, spte with accessed bit is created from gpte at gfnA+indexA. > >>> 2) write to gfnA+indexA, spte has accessed bit set, write_flooding_count > >>> is not increased. > >>> 3) repeat > >>> > >> > >> I think the result is just we hoped, we do not want to zap the shadow page > >> because the spte is currently used by the guest, it also will be used in the > >> next repetition. So do not increase 'write_flooding_count' is a good choice. > > > > Its not used. Step 2) is write to write protected shadow page at > > gfnA. > > > >> Let's consider what will happen if we increase 'write_flooding_count': > >> 1: after three repetitions, zap the shadow page > >> 2: in step 1, we will alloc a new shadow page for gpte at gfnA+indexA > >> 3: in step 2, the flooding count is creased, so after 3 repetitions, the > >> shadow page can be zapped again, repeat 1 to 3. > > > > The shadow page will not be zapped because the spte created from > > gfnA+indexA has the accessed bit set: > > > > if (spte && !(*spte & shadow_accessed_mask)) > > sp->write_flooding_count++; > > else > > sp->write_flooding_count = 0; > > > > Marcelo, i am still confused with your example, in step 3), what is repeated? > it repeats step 2) or it repeats step 1) and 2)? > > Only step 2) is repeated i guess, right? if it is yes, it works well: > when the guest writes gpte, the spte of corresponding shadow page is zapped > (level > 1) or it is speculatively fetched(level == 1), the accessed bit is > cleared in both case. Right. > the later write can detect that the accessed bit is not set, and write_flooding_count > is increased. finally, the shadow page is zapped, the gpte is written directly. > > >> The result is the shadow page for gfnA is alloced and zapped again and again, > >> yes? > > > > The point is you cannot rely on the accessed bit of sptes that have been > > instantiated with the accessed bit set to decide whether or not to zap. > > Because the accessed bit will only be cleared on host memory pressure. > > > > But the accessed bit is also cleared after spte is written. Right. But only one of the 512 sptes. Worst case, a shadow that has 1 spte with accessed bit at every 3 spte entries would not be zapped for a linear write of the entire guest pagetable. The current heuristic does not suffer from this issue. I guess it is OK to be more trigger happy with zapping by ignoring the accessed bit, clearing the flood counter on page fault. -- 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
