Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock

[Sean Christopherson]

On Tue, Jan 26, 2021, Sean Christopherson wrote:
> On Tue, Jan 26, 2021, Paolo Bonzini wrote:
> > On 21/01/21 22:32, Sean Christopherson wrote:
> > > Coming back to this series, I wonder if the RCU approach is truly necessary to
> > > get the desired scalability.  If both zap_collapsible_sptes() and NX huge page
> > > recovery zap_only_  leaf SPTEs, then the only path that can actually unlink a
> > > shadow page while holding the lock for read is the page fault path that installs
> > > a huge page over an existing shadow page.
> > > 
> > > Assuming the above analysis is correct, I think it's worth exploring alternatives
> > > to using RCU to defer freeing the SP memory, e.g. promoting to a write lock in
> > > the specific case of overwriting a SP (though that may not exist for rwlocks),
> > > or maybe something entirely different?
> > 
> > You can do the deferred freeing with a short write-side critical section to
> > ensure all readers have terminated.
> 
> Hmm, the most obvious downside I see is that the zap_collapsible_sptes() case
> will not scale as well as the RCU approach.  E.g. the lock may be heavily
> contested when refaulting all of guest memory to (re)install huge pages after a
> failed migration.
> 
> Though I wonder, could we do something even more clever for that particular
> case?  And I suppose it would apply to NX huge pages as well.  Instead of
> zapping the leaf PTEs and letting the fault handler install the huge page, do an
> in-place promotion when dirty logging is disabled.  That could all be done under
> the read lock, and with Paolo's method for deferred free on the back end.  That
> way only the thread doing the memslot update would take mmu_lock for write, and
> only once per memslot update.

Oh, and we could even skip the remote TLB flush in that case since the GPA->HPA
translation is unchanged.