Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 22, 2013 at 09:34:13AM +0300, Gleb Natapov wrote:
> On Tue, May 21, 2013 at 10:33:30PM -0300, Marcelo Tosatti wrote:
> > On Tue, May 21, 2013 at 11:39:03AM +0300, Gleb Natapov wrote:
> > > > Any pages with stale information will be zapped by kvm_mmu_zap_all().
> > > > When that happens, page faults will take place which will automatically 
> > > > use the new generation number.
> > > > 
> > > > So still not clear why is this necessary.
> > > > 
> > > This is not, strictly speaking, necessary, but it is the sane thing to do.
> > > You cannot update page's generation number to prevent it from been
> > > destroyed since after kvm_mmu_zap_all() completes stale ptes in the
> > > shadow page may point to now deleted memslot. So why build shadow page
> > > table with a page that is in a process of been destroyed?
> > 
> > OK, can this be introduced separately, in a later patch, with separate
> > justification, then?
> > 
> > Xiao please have the first patches of the patchset focus on the problem
> > at hand: fix long mmu_lock hold times.
> > 
> > > Not sure what you mean again. We flush TLB once before entering this function.
> > > kvm_reload_remote_mmus() does this for us, no?
> > 
> > kvm_reload_remote_mmus() is used as an optimization, its separate from the
> > problem solution.
> > 
> > > > 
> > > > What was suggested was... go to phrase which starts with "The only purpose
> > > > of the generation number should be to".
> > > > 
> > > > The comment quoted here does not match that description.
> > > > 
> > > The comment describes what code does and in this it is correct.
> > > 
> > > You propose to not reload roots right away and do it only when root sp
> > > is encountered, right? So my question is what's the point? There are,
> > > obviously, root sps with invalid generation number at this point, so
> > > reload will happen regardless in kvm_mmu_prepare_zap_page(). So why not
> > > do it here right away and avoid it in kvm_mmu_prepare_zap_page() for
> > > invalid and obsolete sps as I proposed in one of my email?
> > 
> > Sure. But Xiao please introduce that TLB collapsing optimization as a
> > later patch, so we can reason about it in a more organized fashion.
> 
> So, if I understand correctly, you are asking to move is_obsolete_sp()
> check from kvm_mmu_get_page() and kvm_reload_remote_mmus() from
> kvm_mmu_invalidate_all_pages() to a separate patch.

- Move the is_obsolete_sp() check from kvm_mmu_get_page() to a separate
optimization patch.
- Move kvm_reload_remote_mmus() optimization to a separate optimization
patch.
- Leave the is_obsolete_sp() check in kvm_mmu_invalidate_all_pages(), as
it is necessary to guarantee forward progress, in the initial patches.

> Fine by me, but if
> we drop kvm_reload_remote_mmus() from kvm_mmu_invalidate_all_pages() the
> call to kvm_mmu_invalidate_all_pages() in emulator_fix_hypercall() will
> become nop. But I question the need to zap all shadow pages tables there
> in the first place, why kvm_flush_remote_tlbs() is not enough?

Good question.