linux-kernel - Re: [PATCH 1/8] KVM: nSVM: Sync next

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Date:   Wed, 20 Apr 2022 17:00:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02

On 4/4/22 19:21, Sean Christopherson wrote:
> On Mon, Apr 04, 2022, Maciej S. Szmigiero wrote:
>>> @@ -1606,7 +1622,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>>>    	nested_copy_vmcb_control_to_cache(svm, ctl);
>>>    	svm_switch_vmcb(svm, &svm->nested.vmcb02);
>>> -	nested_vmcb02_prepare_control(svm);
>>> +	nested_vmcb02_prepare_control(svm, save->rip);
>>
>> 					   ^
>> I guess this should be "svm->vmcb->save.rip", since
>> KVM_{GET,SET}_NESTED_STATE "save" field contains vmcb01 data,
>> not vmcb{0,1}2 (in contrast to the "control" field).
> 
> Argh, yes.  Is userspace required to set L2 guest state prior to KVM_SET_NESTED_STATE?
> If not, this will result in garbage being loaded into vmcb02.
> 

Let's just require X86_FEATURE_NRIPS, either in general or just to
enable nested virtualiazation, i.e.:

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index fc1725b7d05f..f8fc8a1b09f1 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4904,10 +4904,12 @@ static __init int svm_hardware_setup(void)
  			goto err;
  	}
  
-	if (nrips) {
-		if (!boot_cpu_has(X86_FEATURE_NRIPS))
-			nrips = false;
-	}
+	if (!boot_cpu_has(X86_FEATURE_NRIPS))
+		nrips = false;
+	if (nested & !nrips) {
+		pr_warn("Next RIP Save not available, disabling nested virtualization\n");
+		nested = false;
+	}
  
  	enable_apicv = avic = avic && npt_enabled && (boot_cpu_has(X86_FEATURE_AVIC) || force_avic);
  

If I looked it up correctly it was introduced around 2010-2011.

Paolo