[RFC PATCH 11/11] KVM: PPC: Eliminate global spinlock in kvmppc_h_enter

[Marcelo Tosatti]

On Thu, Nov 17, 2011 at 10:55:49AM +1100, Paul Mackerras wrote:
> >From dfd5bcfac841f8a36593edf60d9fb15e0d633287 Mon Sep 17 00:00:00 2001
> From: Paul Mackerras <paulus at samba.org>
> Date: Mon, 14 Nov 2011 13:30:38 +1100
> Subject: 
> 
> Currently, kvmppc_h_enter takes a spinlock that is global to the guest,
> kvm->mmu_lock, in order to check for pending PTE invalidations safely.
> On some workloads, kvmppc_h_enter is called heavily and the use of a
> global spinlock could compromise scalability.  We already use a per-
> guest page spinlock in the form of the bit spinlock on the rmap chain,
> and this gives us synchronization with the PTE invalidation side, which
> also takes the bit spinlock on the rmap chain for each page being
> invalidated.  Thus it is sufficient to check for pending invalidations
> while the rmap chain bit spinlock is held.  However, now we require
> barriers in mmu_notifier_retry() and in the places where
> mmu_notifier_count and mmu_notifier_seq are updated, since we can now
> call mmu_notifier_retry() concurrently with updates to those fields.
> 
> Signed-off-by: Paul Mackerras <paulus at samba.org>
> ---
> Cc'd to kvm at vger.kernel.org for review of the generic kvm changes.

Looks good to me.