linux-kernel - [PATCH 2/2] KVM: x86/mmu: Avoid an extra memslot lookup in try_async

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Date:   Wed, 15 Apr 2020 14:44:14 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.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,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH 2/2] KVM: x86/mmu: Avoid an extra memslot lookup in try_async_pf() for L2

Tweak the L2 vs. private memslot handling in try_async_pf() to avoid an
added memslot lookup and more precisely single out private memslots,
i.e. defer to the common code to handle nonexistent or invalid memslots
to make it clear L2 doesn't require special handling for those cases.

Opportunstically squish a multi-line comment into a single-line comment.

Note, the end result, KVM_PFN_NOSLOT, is unchanged.

Cc: Jim Mattson <jmattson@...gle.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/mmu/mmu.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6d6cb9416179..06d0150ce53b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4082,19 +4082,16 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
 			 gpa_t cr2_or_gpa, kvm_pfn_t *pfn, bool write,
 			 bool *writable)
 {
-	struct kvm_memory_slot *slot;
+	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
 	bool async;
 
-	/*
-	 * Don't expose private memslots to L2.
-	 */
-	if (is_guest_mode(vcpu) && !kvm_is_visible_gfn(vcpu->kvm, gfn)) {
+	/* Don't expose private memslots to L2. */
+	if (is_guest_mode(vcpu) && slot && slot->id >= KVM_USER_MEM_SLOTS) {
 		*pfn = KVM_PFN_NOSLOT;
 		*writable = false;
 		return false;
 	}
 
-	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
 	async = false;
 	*pfn = __gfn_to_pfn_memslot(slot, gfn, false, &async, write, writable);
 	if (!async)
-- 
2.26.0