Re: [patch 3/4] KVM: MMU: reload request from GET_DIRTY_LOG path

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Oct 10, 2014 at 04:09:29PM +0300, Gleb Natapov wrote:
> On Wed, Oct 08, 2014 at 04:22:31PM -0300, Marcelo Tosatti wrote:
> > > > 
> > > > Argh, lets try again:
> > > > 
> > > > skip_pinned = true
> > > > ------------------
> > > > 
> > > > mark page dirty, keep spte intact
> > > > 
> > > > called from get dirty log path.
> > > > 
> > > > skip_pinned = false
> > > > -------------------
> > > > reload remote mmu
> > > > destroy pinned spte.
> > > > 
> > > > called from: dirty log enablement, rmap write protect (unused for pinned
> > > > sptes)
> > > > 
> > > > 
> > > > Note this behaviour is your suggestion:
> > > 
> > > Yes, I remember that and I thought we will not need this skip_pinned
> > > at all.  For rmap write protect case there shouldn't be any pinned pages,
> > > but why dirty log enablement sets skip_pinned to false? Why not mark
> > > pinned pages as dirty just like you do in get dirty log path?
> > 
> > Because if its a large spte, it must be nuked (or marked read-only,
> > which for pinned sptes, is not possible).
> > 
> If a large page has one small page pinned inside it its spte will
> be marked as pinned, correct?  

Correct.

> We did nuke large ptes here until very
> recently: c126d94f2c90ed9d, but we cannot drop a pte here anyway without
> kicking all vcpu from a guest mode, but do you need additional skip_pinned
> parameter?  Why not check if spte is large instead?

Nuke only if large spte is found? Can do that, instead.

> So why not have per slot pinned page list (Xiao suggested the same) and do:

The interface is per-vcpu (that is registration of pinned pages is
performed on a per-vcpu basis).

> spte_write_protect() {
>   if (is_pinned(spte) {
>     if (large(spte))
>        // cannot drop while vcpu are running
>        mmu_reload_pinned_vcpus();
>     else
>        return false;
> }
>     
> 
> get_dirty_log() {
>  for_each(pinned pages i)
>     makr_dirty(i);
> }

That is effectively the same this patchset does, except that the spte
pinned bit is checked at spte_write_protect, instead of looping over 
page pinned list. Fail to see huge advantage there.

I'll drop the skip_pinned parameter and use is_large_pte check instead.

Thanks