[PATCH][for stable/linux-5.15.y] KVM: VMX: Move preemption timer <=> hrtimer dance to common x86

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From: Sean Christopherson <seanjc@xxxxxxxxxx>

commit 98c25ead5eda5e9d41abe57839ad3e8caf19500c upstream.

Handle the switch to/from the hypervisor/software timer when a vCPU is
blocking in common x86 instead of in VMX.  Even though VMX is the only
user of a hypervisor timer, the logic and all functions involved are
generic x86 (unless future CPUs do something completely different and
implement a hypervisor timer that runs regardless of mode).

Handling the switch in common x86 will allow for the elimination of the
pre/post_blocks hooks, and also lets KVM switch back to the hypervisor
timer if and only if it was in use (without additional params).  Add a
comment explaining why the switch cannot be deferred to kvm_sched_out()
or kvm_vcpu_block().

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Message-Id: <20211208015236.1616697-8-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[ta: Fix conflicts in vmx_pre_block and vmx_post_block as per Paolo's
suggestion. Add Reported-by and Link tags.]
Reported-by: syzbot+b6a74be92b5063a0f1ff@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?id=489beb3d76ef14cc6cd18125782dc6f86051a605
Tested-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
 arch/x86/kvm/vmx/vmx.c |  6 ------
 arch/x86/kvm/x86.c     | 21 +++++++++++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9ce45554d637..c95c3675e8d5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7597,17 +7597,11 @@ static int vmx_pre_block(struct kvm_vcpu *vcpu)
 	if (pi_pre_block(vcpu))
 		return 1;
-	if (kvm_lapic_hv_timer_in_use(vcpu))
-		kvm_lapic_switch_to_sw_timer(vcpu);
 	return 0;
 static void vmx_post_block(struct kvm_vcpu *vcpu)
-	if (kvm_x86_ops.set_hv_timer)
-		kvm_lapic_switch_to_hv_timer(vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0622256cd768..5cb4af42ba64 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10043,12 +10043,28 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
+	bool hv_timer;
 	if (!kvm_arch_vcpu_runnable(vcpu) &&
 	    (!kvm_x86_ops.pre_block || static_call(kvm_x86_pre_block)(vcpu) == 0)) {
+		/*
+		 * Switch to the software timer before halt-polling/blocking as
+		 * the guest's timer may be a break event for the vCPU, and the
+		 * hypervisor timer runs only when the CPU is in guest mode.
+		 * Switch before halt-polling so that KVM recognizes an expired
+		 * timer before blocking.
+		 */
+		hv_timer = kvm_lapic_hv_timer_in_use(vcpu);
+		if (hv_timer)
+			kvm_lapic_switch_to_sw_timer(vcpu);
 		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
 		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
+		if (hv_timer)
+			kvm_lapic_switch_to_hv_timer(vcpu);
 		if (kvm_x86_ops.post_block)
@@ -10287,6 +10303,11 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 			r = -EINTR;
 			goto out;
+		/*
+		 * It should be impossible for the hypervisor timer to be in
+		 * use before KVM has ever run the vCPU.
+		 */
+		WARN_ON_ONCE(kvm_lapic_hv_timer_in_use(vcpu));
 		if (kvm_apic_accept_events(vcpu) < 0) {
 			r = 0;

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