linux-kernel - Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE

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Message-ID: <6d7bac03-2270-e908-2e66-1cc4f9425294@redhat.com>
Date:   Thu, 7 Jan 2021 18:51:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
 on nested vmexit

On 07/01/21 18:00, Sean Christopherson wrote:
> Ugh, I assume this is due to one of the "premature" nested_ops->check_events()
> calls that are necessitated by the event mess?  I'm guessing kvm_vcpu_running()
> is the culprit?
> 
> If my assumption is correct, this bug affects nVMX as well.

Yes, though it may be latent.  For SVM it was until we started 
allocating svm->nested on demand.

> Rather than clear the request blindly on any nested VM-Exit, what
> about something like the following?

I think your patch is overkill, KVM_REQ_GET_NESTED_STATE_PAGES is only 
set from KVM_SET_NESTED_STATE so it cannot happen while the VM runs.

Something like this is small enough and works well.

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index a622e63739b4..cb4c6ee10029 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -595,6 +596,8 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
  	svm->nested.vmcb12_gpa = 0;
  	WARN_ON_ONCE(svm->nested.nested_run_pending);

+	kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, &svm->vcpu);
+
  	/* in case we halted in L2 */
  	svm->vcpu.arch.mp_state = KVM_MP_STATE_RUNNABLE;

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index e2f26564a12d..0fbb46990dfc 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4442,6 +4442,8 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 
vm_exit_reason,
  	/* trying to cancel vmlaunch/vmresume is a bug */
  	WARN_ON_ONCE(vmx->nested.nested_run_pending);

+	kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
+
  	/* Service the TLB flush request for L2 before switching to L1. */
  	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
  		kvm_vcpu_flush_tlb_current(vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3f7c1fc7a3ce..b7e784b5489c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8789,7 +8789,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)

  	if (kvm_request_pending(vcpu)) {
  		if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
-			if (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
+			if (WARN_ON_ONCE(!is_guest_mode(&svm->vcpu)))
+				;
+			else if 
(unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
  				r = 0;
  				goto out;
  			}