linux-kernel - Re: [PATCH v3 07/26] KVM: VMX: Move preemption timer <=> hrtimer dance to common x86

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Message-ID: <244097d2-3d14-6031-7733-62be75036d88@redhat.com>
Date:   Wed, 29 Mar 2023 15:47:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tudor Ambarus <tudor.ambarus@...aro.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        Lee Jones <joneslee@...gle.com>
Subject: Re: [PATCH v3 07/26] KVM: VMX: Move preemption timer <=> hrtimer
 dance to common x86

On 3/29/23 14:34, Tudor Ambarus wrote:
> This patch fixes the bug reported at:
> LINK:
> https://syzkaller.appspot.com/bug?id=489beb3d76ef14cc6cd18125782dc6f86051a605
> 
> One may find the strace at:
> LINK:https://syzkaller.appspot.com/text?tag=CrashLog&x=1798b54ec80000
> and the c reproducer at:
> LINK:https://syzkaller.appspot.com/text?tag=ReproC&x=10365781c80000
> 
> Since I've no experience with kvm, it would be helpful if one of you can
> provide some guidance. Do you think it is worth to backport this patch
> to stable (together with its prerequisite patches), or shall I try to
> get familiar with the code and try to provide a less invasive fix?

I think it is enough to fix the conflicts in vmx_pre_block and
vmx_post_block, there are no prerequisites:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0718658268fe..895069038856 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7577,17 +7577,11 @@ 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);
-
  	pi_post_block(vcpu);
  }
  
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fcfa3fedf84f..4eca3ec38afd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10022,12 +10022,28 @@ 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);
  		kvm_vcpu_block(vcpu);
  		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
  
+		if (hv_timer)
+			kvm_lapic_switch_to_hv_timer(vcpu);
+
  		if (kvm_x86_ops.post_block)
  			static_call(kvm_x86_post_block)(vcpu);
  
@@ -10266,6 +10282,11 @@ 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));
  		kvm_vcpu_block(vcpu);
  		if (kvm_apic_accept_events(vcpu) < 0) {
  			r = 0;

The fix is due to the second "if" changing from
kvm_x86_ops.set_hv_timer to hv_timer.

Paolo