From: Ben Gardon <bgardon@google.com>
To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>, Peter Xu <peterx@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Peter Shier <pshier@google.com>,
Peter Feiner <pfeiner@google.com>,
Junaid Shahid <junaids@google.com>,
Jim Mattson <jmattson@google.com>,
Yulei Zhang <yulei.kernel@gmail.com>,
Wanpeng Li <kernellwp@gmail.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
Ben Gardon <bgardon@google.com>
Subject: [PATCH v2 07/13] KVM: x86/mmu: handle cmpxchg failure in kvm_tdp_mmu_get_root
Date: Thu, 1 Apr 2021 16:37:30 -0700 [thread overview]
Message-ID: <20210401233736.638171-8-bgardon@google.com> (raw)
In-Reply-To: <20210401233736.638171-1-bgardon@google.com>
To reduce dependence on the MMU write lock, don't rely on the assumption
that the atomic operation in kvm_tdp_mmu_get_root will always succeed.
By not relying on that assumption, threads do not need to hold the MMU
lock in write mode in order to take a reference on a TDP MMU root.
In the root iterator, this change means that some roots might have to be
skipped if they are found to have a zero refcount. This will still never
happen as of this patch, but a future patch will need that flexibility to
make the root iterator safe under the MMU read lock.
Signed-off-by: Ben Gardon <bgardon@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++-----
arch/x86/kvm/mmu/tdp_mmu.h | 13 +++----------
2 files changed, 9 insertions(+), 15 deletions(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 697ea882a3e4..886bc170f2a5 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -88,10 +88,12 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
next_root = list_first_entry(&kvm->arch.tdp_mmu_roots,
typeof(*next_root), link);
+ while (!list_entry_is_head(next_root, &kvm->arch.tdp_mmu_roots, link) &&
+ !kvm_tdp_mmu_get_root(kvm, next_root))
+ next_root = list_next_entry(next_root, link);
+
if (list_entry_is_head(next_root, &kvm->arch.tdp_mmu_roots, link))
next_root = NULL;
- else
- kvm_tdp_mmu_get_root(kvm, next_root);
if (prev_root)
kvm_tdp_mmu_put_root(kvm, prev_root);
@@ -161,10 +163,9 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
/* Check for an existing root before allocating a new one. */
for_each_tdp_mmu_root(kvm, root, kvm_mmu_role_as_id(role)) {
- if (root->role.word == role.word) {
- kvm_tdp_mmu_get_root(kvm, root);
+ if (root->role.word == role.word &&
+ kvm_tdp_mmu_get_root(kvm, root))
goto out;
- }
}
root = alloc_tdp_mmu_page(vcpu, 0, vcpu->arch.mmu->shadow_root_level);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index 1ec7914ecff9..f0a26214e999 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -7,17 +7,10 @@
hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
-static inline void kvm_tdp_mmu_get_root(struct kvm *kvm,
- struct kvm_mmu_page *root)
+__must_check static inline bool kvm_tdp_mmu_get_root(struct kvm *kvm,
+ struct kvm_mmu_page *root)
{
- lockdep_assert_held_write(&kvm->mmu_lock);
-
- /*
- * This should never fail since roots are removed from the roots
- * list under the MMU write lock when their reference count falls
- * to zero.
- */
- refcount_inc_not_zero(&root->tdp_mmu_root_count);
+ return refcount_inc_not_zero(&root->tdp_mmu_root_count);
}
void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);
--
2.31.0.208.g409f899ff0-goog
next prev parent reply other threads:[~2021-04-01 23:38 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 23:37 [PATCH v2 00/13] More parallel operations for the TDP MMU Ben Gardon
2021-04-01 23:37 ` [PATCH v2 01/13] KVM: x86/mmu: Re-add const qualifier in kvm_tdp_mmu_zap_collapsible_sptes Ben Gardon
2021-05-26 21:25 ` Sean Christopherson
2021-04-01 23:37 ` [PATCH v2 02/13] KVM: x86/mmu: Move kvm_mmu_(get|put)_root to TDP MMU Ben Gardon
2021-04-01 23:37 ` [PATCH v2 03/13] KVM: x86/mmu: use tdp_mmu_free_sp to free roots Ben Gardon
2021-04-01 23:37 ` [PATCH v2 04/13] KVM: x86/mmu: Merge TDP MMU put and free root Ben Gardon
2021-04-01 23:37 ` [PATCH v2 05/13] KVM: x86/mmu: Refactor yield safe root iterator Ben Gardon
2021-04-01 23:37 ` [PATCH v2 06/13] KVM: x86/mmu: Make TDP MMU root refcount atomic Ben Gardon
2021-04-01 23:37 ` Ben Gardon [this message]
2021-04-01 23:37 ` [PATCH v2 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU Ben Gardon
2021-04-01 23:37 ` [PATCH v2 09/13] KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock Ben Gardon
2021-04-02 7:53 ` Paolo Bonzini
2021-04-12 18:21 ` Ben Gardon
2021-04-01 23:37 ` [PATCH v2 10/13] KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU " Ben Gardon
2021-04-02 11:14 ` Paolo Bonzini
2021-04-01 23:37 ` [PATCH v2 11/13] KVM: x86/mmu: Allow enabling / disabling dirty logging under " Ben Gardon
2021-04-01 23:37 ` [PATCH v2 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU Ben Gardon
2021-04-01 23:37 ` [PATCH v2 13/13] KVM: x86/mmu: Tear down roots in fast invalidation thread Ben Gardon
2021-04-02 11:43 ` [PATCH v2 00/13] More parallel operations for the TDP MMU Paolo Bonzini
2021-05-26 21:34 ` Sean Christopherson
2021-05-27 11:41 ` Paolo Bonzini
2021-05-27 15:26 ` Sean Christopherson
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