Re: [PATCH 0/2] KVM: x86/mmu: Minor cleanup in try_async_pf()

[Paolo Bonzini]

On 15/04/20 23:44, Sean Christopherson wrote:
> 
> I'm not 100% on whether or not open coding the private memslot check in
> patch 2 is a good idea.  Avoiding the extra memslot lookup is nice, but
> that could be done by providing e.g. kvm_is_memslot_visible(). 

Yeah, that's better.  The patch is so small that it's even pointless to
split it in two:

From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Subject: [PATCH] KVM: x86/mmu: Avoid an extra memslot lookup in try_async_pf() for L2

Create a new function kvm_is_visible_memslot() and use it from
kvm_is_visible_gfn(); use the new function in try_async_pf() too,
to avoid an extra memslot lookup.

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6d6cb9416179..fe04ce843a57 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4082,19 +4082,18 @@ 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)) {
+	if (is_guest_mode(vcpu) && !kvm_is_visible_memslot(slot)) {
 		*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)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 658215f6102c..7d4f1eb70274 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1357,6 +1357,12 @@ static inline void kvm_vcpu_set_dy_eligible(struct kvm_vcpu *vcpu, bool val)
 }
 #endif /* CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT */
 
+static inline bool kvm_is_visible_memslot(struct kvm_memory_slot *memslot)
+{
+	return (memslot && memslot->id < KVM_USER_MEM_SLOTS &&
+		!(memslot->flags & KVM_MEMSLOT_INVALID));
+}
+
 struct kvm_vcpu *kvm_get_running_vcpu(void);
 struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index da8fd45e0e3e..8aa577db131e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1607,11 +1607,7 @@ bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
 {
 	struct kvm_memory_slot *memslot = gfn_to_memslot(kvm, gfn);
 
-	if (!memslot || memslot->id >= KVM_USER_MEM_SLOTS ||
-	      memslot->flags & KVM_MEMSLOT_INVALID)
-		return false;
-
-	return true;
+	return kvm_is_visible_memslot(memslot);
 }
 EXPORT_SYMBOL_GPL(kvm_is_visible_gfn);