linux-kernel - [PATCH 1/2] KVM: x86/mmu: Remove dead code in .change

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Date:   Tue,  8 Aug 2023 16:53:06 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com,
        Yan Zhao <yan.y.zhao@...el.com>
Subject: [PATCH 1/2] KVM: x86/mmu: Remove dead code in .change_pte() handler in x86 TDP MMU

Remove the dead code set_spte_gfn() in x86 TDP MMU's .change_pte() handler
to save CPU cycles and to prepare for the optimization in next patch.

As explained in commit c13fda237f08 ("KVM: Assert that notifier count is
elevated in .change_pte()"),

when .change_pte() was added by commit 828502d30073 ("ksm: add mmu_notifier
set_pte_at_notify()"), .change_pte() was invoked without any surrounding
notifications;

However, since commit 6bdb913f0a70 ("mm: wrap calls to set_pte_at_notify
with invalidate_range_start and invalidate_range_end"), all calls to
.change_pte() are guaranteed to be surrounded by .invalidate_range_start()
and .invalidate_range_end() pair.

As .invalidate_range_start() will always cause KVM to zap related SPTE, and
page fault path will not install new SPTEs successfully before
.invalidate_range_end(), kvm_set_spte_gfn() should not be able to find any
shadow present leaf entries to operate on and therefore set_spte_gfn()
is never called any more.

So, in TDP MMU, just drop the set_spte_gfn() and only keep warning of huge
pages.

Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 40 ++++----------------------------------
 1 file changed, 4 insertions(+), 36 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6250bd3d20c1..89a1f222e823 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1235,36 +1235,6 @@ bool kvm_tdp_mmu_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 	return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn);
 }
 
-static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
-			 struct kvm_gfn_range *range)
-{
-	u64 new_spte;
-
-	/* Huge pages aren't expected to be modified without first being zapped. */
-	WARN_ON(pte_huge(range->arg.pte) || range->start + 1 != range->end);
-
-	if (iter->level != PG_LEVEL_4K ||
-	    !is_shadow_present_pte(iter->old_spte))
-		return false;
-
-	/*
-	 * Note, when changing a read-only SPTE, it's not strictly necessary to
-	 * zero the SPTE before setting the new PFN, but doing so preserves the
-	 * invariant that the PFN of a present * leaf SPTE can never change.
-	 * See handle_changed_spte().
-	 */
-	tdp_mmu_iter_set_spte(kvm, iter, 0);
-
-	if (!pte_write(range->arg.pte)) {
-		new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,
-								  pte_pfn(range->arg.pte));
-
-		tdp_mmu_iter_set_spte(kvm, iter, new_spte);
-	}
-
-	return true;
-}
-
 /*
  * Handle the changed_pte MMU notifier for the TDP MMU.
  * data is a pointer to the new pte_t mapping the HVA specified by the MMU
@@ -1273,12 +1243,10 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
  */
 bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 {
-	/*
-	 * No need to handle the remote TLB flush under RCU protection, the
-	 * target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
-	 * shadow page. See the WARN on pfn_changed in handle_changed_spte().
-	 */
-	return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
+	/* Huge pages aren't expected to be modified */
+	WARN_ON(pte_huge(range->arg.pte) || range->start + 1 != range->end);
+
+	return false;
 }
 
 /*
-- 
2.17.1