linux-kernel - [PATCH 1/2] KVM: x86: Remove obsolete disabling of page faults in kvm_arch_vcpu

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Message-Id: <20210123000334.3123628-2-seanjc@google.com>
Date:   Fri, 22 Jan 2021 16:03:33 -0800
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
Subject: [PATCH 1/2] KVM: x86: Remove obsolete disabling of page faults in kvm_arch_vcpu_put()

Remove the disabling of page faults across kvm_steal_time_set_preempted()
as KVM now accesses the steal time struct (shared with the guest) via a
cached mapping (see commit b043138246a4, "x86/KVM: Make sure
KVM_VCPU_FLUSH_TLB flag is not missed".)  The cache lookup is flagged as
atomic, thus it would be a bug if KVM tried to resolve a new pfn, i.e.
we want the splat that would be reached via might_fault().

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9a8969a6dd06..3f4b09d9f25b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4031,15 +4031,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	if (vcpu->preempted && !vcpu->arch.guest_state_protected)
 		vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);
 
-	/*
-	 * Disable page faults because we're in atomic context here.
-	 * kvm_write_guest_offset_cached() would call might_fault()
-	 * that relies on pagefault_disable() to tell if there's a
-	 * bug. NOTE: the write to guest memory may not go through if
-	 * during postcopy live migration or if there's heavy guest
-	 * paging.
-	 */
-	pagefault_disable();
 	/*
 	 * kvm_memslots() will be called by
 	 * kvm_write_guest_offset_cached() so take the srcu lock.
@@ -4047,7 +4038,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	idx = srcu_read_lock(&vcpu->kvm->srcu);
 	kvm_steal_time_set_preempted(vcpu);
 	srcu_read_unlock(&vcpu->kvm->srcu, idx);
-	pagefault_enable();
 	kvm_x86_ops.vcpu_put(vcpu);
 	vcpu->arch.last_host_tsc = rdtsc();
 	/*
-- 
2.30.0.280.ga3ce27912f-goog