Re: [PATCH v4 03/23] KVM: PPC: Flush/zap all memslots on kvm_arch_flush_shadow_all()

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 04, 2026, Narayana Murty N wrote:
> Hi Sean,
> 
> On 31/10/26 2:50 AM, Sean Christopherson wrote:
> > Flush/zap every memslot in kvm_arch_flush_shadow_all(), i.e. when the
> > owning userspace process is exiting, as KVM must ensure all associated
> > memory is unmapped from the guest.
> > 
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >   arch/powerpc/include/asm/kvm_host.h |  1 -
> >   arch/powerpc/kvm/powerpc.c          | 11 +++++++++++
> >   2 files changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> > index 708a32f89776..50a42f80e5f1 100644
> > --- a/arch/powerpc/include/asm/kvm_host.h
> > +++ b/arch/powerpc/include/asm/kvm_host.h
> > @@ -910,7 +910,6 @@ struct kvm_vcpu_arch {
> >   #define __KVM_HAVE_CREATE_DEVICE
> >   static inline void kvm_arch_memslots_updated(struct kvm *kvm, u64 gen) {}
> > -static inline void kvm_arch_flush_shadow_all(struct kvm *kvm) {}
> >   static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
> >   static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index 9194cf492d1c..9e9d0865c2f6 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -764,6 +764,17 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >   	kvmppc_core_commit_memory_region(kvm, old, new, change);
> >   }
> > +void kvm_arch_flush_shadow_all(struct kvm *kvm)
> > +{
> > +	struct kvm_memory_slot *slot;
> > +	struct kvm_memslots *slots;
> > +	int bkt;
> > +
> > +	slots = kvm_memslots(kvm);
> > +	kvm_for_each_memslot(slot, bkt, slots)
> > +		kvmppc_core_flush_memslot(kvm, slot);
> 
> For Book3S HV/hash, I am not sure the existing flush_memslot helper can
> be used on an active memslot like this.
> 
> The normal memslot invalidation path seems to deliberately establish
> the opposite ordering.  kvm_invalidate_memslot() first installs an
> INVALID slot and makes it active:
> 
> 	invalid_slot->flags |= KVM_MEMSLOT_INVALID;
> 	kvm_replace_memslot(kvm, old, invalid_slot);
> 	kvm_swap_active_memslots(kvm, old->as_id);
> and only then does kvm_arch_flush_shadow_memslot(kvm, old);.
> 
> The comment there explicitly states:
> /*
>  * From this point no new shadow pages pointing to a deleted,
>  * or moved, memslot will be created.
>  */
> 
> Similarly, kvmppc_core_flush_memslot_hv() in
> arch/powerpc/kvm/book3s_64_mmu_hv.c relies on the memslot already being
> invalid when testing KVMPPC_RMAP_PRESENT without locking, as no new HPTEs
> can be created.
> 
> However, kvm_arch_flush_shadow_all() in arch/powerpc/kvm/powerpc.c now
> invokes the same helper while walking active memslots.
> 
> Is there any serialization in the mmu_notifier.release() path that provides
> the same guarantee against new HPTE creation?

There can't be any outstanding *usable* references to vCPUs at this point.
mmu_notifier.release() is only invoked if the process is exiting or the VM is
dying (which requires the associated file to have been released).

If the process is existing, KVM_RUN (and most ioctls) are unreachable due to the
"dev->kvm->mm != current->mm" checks.  Not to mention that if the process is
exiting, doing anything with a memslot is completely broken because it means
things like hva_to_pfn() are somehow operating on the wrong mm.

If the VM is dying, there can't be any references to vCPU FDs, because each vCPU
holds a reference to the VM.

> Otherwise, couldn't an H_ENTER or fault path create a new HPTE while the
> rmap is being scanned, or after it has already been examined?