From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock
Date: Wed, 11 Aug 2021 15:52:13 +0000 [thread overview]
Message-ID: <YRPyLagRbw5QKoNc@google.com> (raw)
In-Reply-To: <74bb6910-4a0c-4d2f-e6b5-714a3181638e@redhat.com>
On Wed, Aug 11, 2021, Paolo Bonzini wrote:
> On 11/08/21 00:45, Sean Christopherson wrote:
> > Use an entirely new spinlock even though piggybacking tdp_mmu_pages_lock
> > would functionally be ok. Usurping the lock could degrade performance when
> > building upper level page tables on different vCPUs, especially since the
> > unsync flow could hold the lock for a comparatively long time depending on
> > the number of indirect shadow pages and the depth of the paging tree.
>
> If we are to introduce a new spinlock, do we need to make it conditional and
> pass it around like this? It would be simpler to just take it everywhere
> (just like, in patch 2, passing "shared == true" to tdp_mmu_link_page is
> always safe anyway).
It's definitely not necessary to pass it around. I liked this approach because
the lock is directly referenced only by the TDP MMU.
My runner up was to key off of is_tdp_mmu_enabled(), which is not strictly
necessary, but I didn't like checking is_tdp_mmu() this far down the call chain.
E.g. minus comments and lockdeps
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d574c68cbc5c..651256a10cb9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2594,6 +2594,8 @@ static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
*/
int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
{
+ bool tdp_mmu = is_tdp_mmu_enabled(vcpu->kvm);
+ bool write_locked = !tdp_mmu;
struct kvm_mmu_page *sp;
/*
@@ -2617,9 +2619,19 @@ int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
if (sp->unsync)
continue;
+ if (!write_locked) {
+ write_locked = true;
+ spin_lock(&vcpu->kvm->arch.tdp_mmu_unsync_pages_lock);
+
+ if (READ_ONCE(sp->unsync))
+ continue;
+ }
+
WARN_ON(sp->role.level != PG_LEVEL_4K);
kvm_unsync_page(vcpu, sp);
}
+ if (tdp_mmu && write_locked)
+ spin_unlock(&vcpu->kvm->arch.tdp_mmu_unsync_pages_lock);
/*
* We need to ensure that the marking of unsync pages is visible
All that said, I do not have a strong preference. Were you thinking something
like this?
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d574c68cbc5c..b622e8a13b8b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2595,6 +2595,7 @@ static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
{
struct kvm_mmu_page *sp;
+ bool locked = false;
/*
* Force write-protection if the page is being tracked. Note, the page
@@ -2617,9 +2618,34 @@ int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
if (sp->unsync)
continue;
+ /*
+ * TDP MMU page faults require an additional spinlock as they
+ * run with mmu_lock held for read, not write, and the unsync
+ * logic is not thread safe. Take the spinklock regardless of
+ * the MMU type to avoid extra conditionals/parameters, there's
+ * no meaningful penalty if mmu_lock is held for write.
+ */
+ if (!locked) {
+ locked = true;
+ spin_lock(&kvm->arch.mmu_unsync_pages_lock);
+
+ /*
+ * Recheck after taking the spinlock, a different vCPU
+ * may have since marked the page unsync. A false
+ * positive on the unprotected check above is not
+ * possible as clearing sp->unsync _must_ hold mmu_lock
+ * for write, i.e. unsync cannot transition from 0->1
+ * while this CPU holds mmu_lock for read.
+ */
+ if (READ_ONCE(sp->unsync))
+ continue;
+ }
+
WARN_ON(sp->role.level != PG_LEVEL_4K);
kvm_unsync_page(vcpu, sp);
}
+ if (locked)
+ spin_unlock(&kvm->arch.mmu_unsync_pages_lock);
/*
* We need to ensure that the marking of unsync pages is visible
next prev parent reply other threads:[~2021-08-11 15:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-10 22:45 [PATCH 0/2] KVM: x86/mmu: Fix unsync races within TDP MMU Sean Christopherson
2021-08-10 22:45 ` [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock Sean Christopherson
2021-08-11 11:47 ` Paolo Bonzini
2021-08-11 15:52 ` Sean Christopherson [this message]
2021-08-12 15:37 ` Paolo Bonzini
2021-08-12 16:06 ` Sean Christopherson
2021-08-10 22:45 ` [PATCH 2/2] KVM: x86/mmu: Drop 'shared' param from tdp_mmu_link_page() Sean Christopherson
2021-08-11 16:33 ` Ben Gardon
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