KVM: delete .change_pte MMU notifier callback

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>
Acked-by: Anup Patel <anup@brainfault.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Message-ID: <20240405115815.3226315-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 0 insertions, 43 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fb49c2a60200..fb0f922d3109 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -705,48 +705,6 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
 	return __kvm_handle_hva_range(kvm, &range).ret;
 }
 
-static bool kvm_change_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
-{
-	/*
-	 * Skipping invalid memslots is correct if and only change_pte() is
-	 * surrounded by invalidate_range_{start,end}(), which is currently
-	 * guaranteed by the primary MMU.  If that ever changes, KVM needs to
-	 * unmap the memslot instead of skipping the memslot to ensure that KVM
-	 * doesn't hold references to the old PFN.
-	 */
-	WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
-
-	if (range->slot->flags & KVM_MEMSLOT_INVALID)
-		return false;
-
-	return kvm_set_spte_gfn(kvm, range);
-}
-
-static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
-					struct mm_struct *mm,
-					unsigned long address,
-					pte_t pte)
-{
-	struct kvm *kvm = mmu_notifier_to_kvm(mn);
-	const union kvm_mmu_notifier_arg arg = { .pte = pte };
-
-	trace_kvm_set_spte_hva(address);
-
-	/*
-	 * .change_pte() must be surrounded by .invalidate_range_{start,end}().
-	 * If mmu_invalidate_in_progress is zero, then no in-progress
-	 * invalidations, including this one, found a relevant memslot at
-	 * start(); rechecking memslots here is unnecessary.  Note, a false
-	 * positive (count elevated by a different invalidation) is sub-optimal
-	 * but functionally ok.
-	 */
-	WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
-	if (!READ_ONCE(kvm->mmu_invalidate_in_progress))
-		return;
-
-	kvm_handle_hva_range(mn, address, address + 1, arg, kvm_change_spte_gfn);
-}
-
 void kvm_mmu_invalidate_begin(struct kvm *kvm)
 {
 	lockdep_assert_held_write(&kvm->mmu_lock);
@@ -964,7 +922,6 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
 	.clear_flush_young	= kvm_mmu_notifier_clear_flush_young,
 	.clear_young		= kvm_mmu_notifier_clear_young,
 	.test_young		= kvm_mmu_notifier_test_young,
-	.change_pte		= kvm_mmu_notifier_change_pte,
 	.release		= kvm_mmu_notifier_release,
 };