KVM: arm/arm64: Skip updating PMD entry if no change

Contention on updating a PMD entry by a large number of vcpus can lead
to duplicate work when handling stage 2 page faults. As the page table
update follows the break-before-make requirement of the architecture,
it can lead to repeated refaults due to clearing the entry and
flushing the tlbs.

This problem is more likely when -

* there are large number of vcpus
* the mapping is large block mapping

such as when using PMD hugepages (512MB) with 64k pages.

Fix this by skipping the page table update if there is no change in
the entry being updated.

Cc: stable@vger.kernel.org
Fixes: ad361f09 ("KVM: ARM: Support hugetlbfs backed huge pages")
Reviewed-by: NSuzuki Poulose <suzuki.poulose@arm.com>
Acked-by: NChristoffer Dall <christoffer.dall@arm.com>
Signed-off-by: NPunit Agrawal <punit.agrawal@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

virt/kvm/arm/mmu.c

@@ -1044,19 +1044,35 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
 	pmd = stage2_get_pmd(kvm, cache, addr);
 	VM_BUG_ON(!pmd);
 
-	/*
-	 * Mapping in huge pages should only happen through a fault.  If a
-	 * page is merged into a transparent huge page, the individual
-	 * subpages of that huge page should be unmapped through MMU
-	 * notifiers before we get here.
-	 *
-	 * Merging of CompoundPages is not supported; they should become
-	 * splitting first, unmapped, merged, and mapped back in on-demand.
-	 */
-	VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
-
 	old_pmd = *pmd;
 	if (pmd_present(old_pmd)) {
+		/*
+		 * Multiple vcpus faulting on the same PMD entry, can
+		 * lead to them sequentially updating the PMD with the
+		 * same value. Following the break-before-make
+		 * (pmd_clear() followed by tlb_flush()) process can
+		 * hinder forward progress due to refaults generated
+		 * on missing translations.
+		 *
+		 * Skip updating the page table if the entry is
+		 * unchanged.
+		 */
+		if (pmd_val(old_pmd) == pmd_val(*new_pmd))
+			return 0;
+
+		/*
+		 * Mapping in huge pages should only happen through a
+		 * fault.  If a page is merged into a transparent huge
+		 * page, the individual subpages of that huge page
+		 * should be unmapped through MMU notifiers before we
+		 * get here.
+		 *
+		 * Merging of CompoundPages is not supported; they
+		 * should become splitting first, unmapped, merged,
+		 * and mapped back in on-demand.
+		 */
+		VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
+
 		pmd_clear(pmd);
 		kvm_tlb_flush_vmid_ipa(kvm, addr);
 	} else {