KVM: Allow arch code to elide TLB flushes when aging a young page

Add a Kconfig to allow architectures to opt-out of a TLB flush when a
young page is aged, as invalidating TLB entries is not functionally
required on most KVM-supported architectures.  Stale TLB entries can
result in false negatives and theoretically lead to suboptimal reclaim,
but in practice all observations have been that the performance gained by
skipping TLB flushes outweighs any performance lost by reclaiming hot
pages.

E.g. the primary MMUs for x86 RISC-V, s390, and PPC Book3S elide the TLB
flush for ptep_clear_flush_young(), and arm64's MMU skips the trailing DSB
that's required for ordering (presumably because there are optimizations
related to eliding other TLB flushes when doing make-before-break).

Link: https://lore.kernel.org/r/20241011021051.1557902-18-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

virt/kvm/Kconfig

@@ -100,6 +100,10 @@ config KVM_GENERIC_MMU_NOTIFIER
        select MMU_NOTIFIER
        bool
 
+config KVM_ELIDE_TLB_FLUSH_IF_YOUNG
+       depends on KVM_GENERIC_MMU_NOTIFIER
+       bool
+
 config KVM_GENERIC_MEMORY_ATTRIBUTES
        depends on KVM_GENERIC_MMU_NOTIFIER
        bool

virt/kvm/kvm_main.c

@@ -630,15 +630,16 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
 static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
 						unsigned long start,
 						unsigned long end,
-						gfn_handler_t handler)
+						gfn_handler_t handler,
+						bool flush_on_ret)
 {
 	struct kvm *kvm = mmu_notifier_to_kvm(mn);
 	const struct kvm_mmu_notifier_range range = {
 		.start		= start,
 		.end		= end,
 		.handler	= handler,
 		.on_lock	= (void *)kvm_null_fn,
-		.flush_on_ret	= true,
+		.flush_on_ret	= flush_on_ret,
 		.may_block	= false,
 	};
 
@@ -650,17 +651,7 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
 							 unsigned long end,
 							 gfn_handler_t handler)
 {
-	struct kvm *kvm = mmu_notifier_to_kvm(mn);
-	const struct kvm_mmu_notifier_range range = {
-		.start		= start,
-		.end		= end,
-		.handler	= handler,
-		.on_lock	= (void *)kvm_null_fn,
-		.flush_on_ret	= false,
-		.may_block	= false,
-	};
-
-	return __kvm_handle_hva_range(kvm, &range).ret;
+	return kvm_handle_hva_range(mn, start, end, handler, false);
 }
 
 void kvm_mmu_invalidate_begin(struct kvm *kvm)
@@ -825,7 +816,8 @@ static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
 {
 	trace_kvm_age_hva(start, end);
 
-	return kvm_handle_hva_range(mn, start, end, kvm_age_gfn);
+	return kvm_handle_hva_range(mn, start, end, kvm_age_gfn,
+				    !IS_ENABLED(CONFIG_KVM_ELIDE_TLB_FLUSH_IF_YOUNG));
 }
 
 static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,