[PATCH v2 10/10] KVM: x86: deprecate KVM_ASYNC_PF_SEND_ALWAYS

[Vitaly Kuznetsov]

Concerns were expressed around APF events delivery when CPU is not
in user mode (KVM_ASYNC_PF_SEND_ALWAYS), e.g.
https://lore.kernel.org/kvm/ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@xxxxxxxxxx/

'Page ready' events are already free from '#PF abuse' but 'page not ready'
notifications still go through #PF (to be changed in the future). Make the
probability of running into issues when APF collides with regular #PF lower
by deprecating KVM_ASYNC_PF_SEND_ALWAYS. The feature doesn't seem to be
important enough for any particular workload to notice the difference.

Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h      |  1 -
 arch/x86/include/uapi/asm/kvm_para.h |  2 +-
 arch/x86/kernel/kvm.c                |  3 ---
 arch/x86/kvm/x86.c                   | 13 +++++++++----
 4 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 356c02bfa587..f491214b7667 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -769,7 +769,6 @@ struct kvm_vcpu_arch {
 		u64 msr_int_val; /* MSR_KVM_ASYNC_PF_INT */
 		u16 vec;
 		u32 id;
-		bool send_user_only;
 		u32 host_apf_flags;
 		unsigned long nested_apf_token;
 		bool delivery_as_pf_vmexit;
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 812e9b4c1114..3cae0faac2b8 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -82,7 +82,7 @@ struct kvm_clock_pairing {
 #define KVM_MAX_MMU_OP_BATCH           32
 
 #define KVM_ASYNC_PF_ENABLED			(1 << 0)
-#define KVM_ASYNC_PF_SEND_ALWAYS		(1 << 1)
+#define KVM_ASYNC_PF_SEND_ALWAYS		(1 << 1) /* deprecated */
 #define KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT	(1 << 2)
 #define KVM_ASYNC_PF_DELIVERY_AS_INT		(1 << 3)
 
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 79730eaef1e1..add123302122 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -324,9 +324,6 @@ static void kvm_guest_cpu_init(void)
 	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_INT) && kvmapf) {
 		u64 pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));
 
-#ifdef CONFIG_PREEMPTION
-		pa |= KVM_ASYNC_PF_SEND_ALWAYS;
-#endif
 		pa |= KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
 
 		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cc1bf6cfc5e0..8222133bf5be 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2648,7 +2648,10 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 {
 	gpa_t gpa = data & ~0x3f;
 
-	/* Bits 4:5 are reserved, Should be zero */
+	/*
+	 * Bits 4:5 are reserved and should be zero. Bit 1
+	 * (KVM_ASYNC_PF_SEND_ALWAYS) was deprecated and is being ignored.
+	 */
 	if (data & 0x30)
 		return 1;
 
@@ -2664,7 +2667,6 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 					sizeof(u64)))
 		return 1;
 
-	vcpu->arch.apf.send_user_only = !(data & KVM_ASYNC_PF_SEND_ALWAYS);
 	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
 	kvm_async_pf_wakeup_all(vcpu);
@@ -10433,8 +10435,11 @@ static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
 	if (!vcpu->arch.apf.delivery_as_pf_vmexit && is_guest_mode(vcpu))
 		return false;
 
-	if (!kvm_pv_async_pf_enabled(vcpu) ||
-	    (vcpu->arch.apf.send_user_only && kvm_x86_ops.get_cpl(vcpu) == 0))
+	/*
+	 * 'Page not present' APF events are only delivered when CPU is in
+	 * user mode and APF mechanism is enabled.
+	 */
+	if (!kvm_pv_async_pf_enabled(vcpu) || kvm_x86_ops.get_cpl(vcpu) == 0)
 		return false;
 
 	return true;
-- 
2.25.4