linux-kernel - [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root

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Message-Id: <20211120045046.3940942-11-seanjc@google.com>
Date:   Sat, 20 Nov 2021 04:50:28 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for
 defunct TDP MMU root

Allow yielding when zapping SPTEs for a defunct TDP MMU root.  Yielding
is safe from a TDP perspective, as the root is unreachable.  The only
potential danger is putting a root from a non-preemptible context, and
KVM currently does not do so.

Yield-unfriendly iteration uses for_each_tdp_mmu_root(), which doesn't
take a reference to each root (it requires mmu_lock be held for the
entire duration of the walk).

tdp_mmu_next_root() is used only by the yield-friendly iterator.

kvm_tdp_mmu_zap_invalidated_roots() is explicitly yield friendly.

kvm_mmu_free_roots() => mmu_free_root_page() is a much bigger fan-out,
but is still yield-friendly in all call sites, as all callers can be
traced back to some combination of vcpu_run(), kvm_destroy_vm(), and/or
kvm_create_vm().

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 3086c6dc74fb..138c7dc41d2c 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -79,6 +79,11 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
 	tdp_mmu_free_sp(sp);
 }
 
+/*
+ * Note, putting a root might sleep, i.e. the caller must have IRQs enabled and
+ * must not explicitly disable preemption (it will be disabled by virtue of
+ * holding mmu_lock, hence the lack of a might_sleep()).
+ */
 void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
 			  bool shared)
 {
@@ -101,7 +106,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
 	 * intermediate paging structures, that may be zapped, as such entries
 	 * are associated with the ASID on both VMX and SVM.
 	 */
-	(void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
+	(void)zap_gfn_range(kvm, root, 0, -1ull, true, false, shared);
 
 	call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
 }
-- 
2.34.0.rc2.393.gf8c9666880-goog