Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback

Paolo Bonzini [off-list ref] writes:

The .change_pte() MMU notifier callback was intended as an
optimization. The original point of it was that KSM could tell KVM to flip
its secondary PTE to a new location without having to first zap it. At
the time there was also an .invalidate_page() callback; both of them were
*not* bracketed by calls to mmu_notifier_invalidate_range_{start,end}(),
and .invalidate_page() also doubled as a fallback implementation of
.change_pte().

Later on, however, both callbacks were changed to occur within an
invalidate_range_start/end() block.

In the case of .change_pte(), commit 6bdb913f0a70 ("mm: wrap calls to
set_pte_at_notify with invalidate_range_start and invalidate_range_end",
2012-10-09) did so to remove the fallback from .invalidate_page() to
.change_pte() and allow sleepable .invalidate_page() hooks.

This however made KVM's usage of the .change_pte() callback completely
moot, because KVM unmaps the sPTEs during .invalidate_range_start()
and therefore .change_pte() has no hope of finding a sPTE to change.
Drop the generic KVM code that dispatches to kvm_set_spte_gfn(), as
well as all the architecture specific implementations.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/arm64/kvm/mmu.c                  | 34 -----------------
 arch/loongarch/include/asm/kvm_host.h |  1 -
 arch/loongarch/kvm/mmu.c              | 32 ----------------
 arch/mips/kvm/mmu.c                   | 30 ---------------
 arch/powerpc/include/asm/kvm_ppc.h    |  1 -
 arch/powerpc/kvm/book3s.c             |  5 ---
 arch/powerpc/kvm/book3s.h             |  1 -
 arch/powerpc/kvm/book3s_64_mmu_hv.c   | 12 ------
 arch/powerpc/kvm/book3s_hv.c          |  1 -
 arch/powerpc/kvm/book3s_pr.c          |  7 ----
 arch/powerpc/kvm/e500_mmu_host.c      |  6 ---

LGTM.

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

cheers