Re: [PATCH 3/6] KVM MMU: optimize/cleanup for marking parent unsync

Marcelo Tosatti
2010-04-14T16:35:44Z

On Wed, Apr 14, 2010 at 11:23:38AM +0800, Xiao Guangrong wrote:
>
>
> Marcelo Tosatti wrote:
>
> >>> I'd prefer to not touch it.
> >> This patch avoids walk all parents and i think this overload is really unnecessary.
> >> It has other tricks in this codepath but i not noticed? :-)
> >
> > My point is that there is no point in optimizing something unless its
> > performance sensitive.
>
> Hi Marcelo,
>
> I think optimizing not only means 'performance' but also means 'smaller code'(maybe 'cleanup'
> is more suitable) and 'logic optimize'(do little things), i'm not sure this patch whether can
> improve system performance obviously but it optimize the code logic and reduce code size, and
> it not harm other code and system performance, right? :-)

Right, but this walking code already is compact and stable. Removing the
unused code variables/definitions is fine, but i'd prefer to not change
the logic just for the sake of code reduction.

> Actually, the origin code has a bug, the code segment in mmu_parent_walk():
>
> | if (!sp->multimapped && sp->parent_pte) {
> | ......
> | return;
> | }
> | hlist_for_each_entry(pte_chain, node, &sp->parent_ptes, link)
> | for (i = 0; i < NR_PTE_CHAIN_ENTRIES; ++i) {
> | ......
> | }
>
> So, if sp->parent_pte == NULL, it's unsafe...
>
> > And as i recall, mmu_unsync_walk was much more
> > sensitive performance wise than parent walking. Actually, gfn_to_memslot
> > seems more important since its also noticeable on EPT/NPT hosts.
>
> Yeah, i also noticed these and i'm looking into these code.

Great.