linux-kernel - [PATCH v7 16/18] KVM: x86/mmu: Enable has_fast

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Message-ID: <20240926013506.860253-17-jthoughton@google.com>
Date: Thu, 26 Sep 2024 01:35:04 +0000
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, James Houghton <jthoughton@...gle.com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v7 16/18] KVM: x86/mmu: Enable has_fast_aging

Because the x86 MMU locklessly implements fast_only versions of
kvm_test_age_gfn and kvm_age_gfn, we can advertise support for
has_fast_aging to allow MGLRU to quickly find better eviction
candidates.

There is one case where the MMU is not 100% accurate: for the shadow
MMU, when A/D bits are not in use, young sptes will never be aged with
the fast_only kvm_age_gfn. In this case, such pages will consistently
appear young, so they will be the least likely eviction candidates.

Signed-off-by: James Houghton <jthoughton@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f47bd88b55e3..1798e3853d27 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7708,6 +7708,8 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
 {
 	int err;
 
+	kvm->mmu_notifier.has_fast_aging = true;
+
 	if (nx_hugepage_mitigation_hard_disabled)
 		return 0;
 
-- 
2.46.0.792.g87dc391469-goog