linux-kernel - Re: [PATCH 3/5] KVM: nVMX: Update VMX controls MSR according to guest CPUID after setting VMX MSRs

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Message-ID: <48fd0fd9-97b4-471e-d8e8-628f51dcdeff@redhat.com>
Date:   Fri, 11 Sep 2020 19:09:27 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Chenyi Qiang <chenyi.qiang@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] KVM: nVMX: Update VMX controls MSR according to guest
 CPUID after setting VMX MSRs

On 02/09/20 20:32, Jim Mattson wrote:
> 
> /* If not VM_EXIT_CLEAR_BNDCFGS, the L2 value propagates to L1.  */
> if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
>         vmcs_write64(GUEST_BNDCFGS, 0);
> 
> BTW, where does the L2 value propagate to L1 if not VM_EXIT_CLEAR_BNDCFGS?

Hmm, nowhere. :/  Probably something like this (not really thought through):

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 1e903d51912b..aba76aa99465 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3317,7 +3317,8 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 	if (!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
 		vmx->nested.vmcs01_debugctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
 	if (kvm_mpx_supported() &&
-		!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS))
+	    (!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS) ||
+	     !(vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)))
 		vmx->nested.vmcs01_guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
 
 	/*
@@ -4186,9 +4187,12 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	vmcs_write32(GUEST_IDTR_LIMIT, 0xFFFF);
 	vmcs_write32(GUEST_GDTR_LIMIT, 0xFFFF);
 
-	/* If not VM_EXIT_CLEAR_BNDCFGS, the L2 value propagates to L1.  */
-	if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
-		vmcs_write64(GUEST_BNDCFGS, 0);
+	if (kvm_mpx_supported()) {
+		if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
+			vmcs_write64(GUEST_BNDCFGS, 0);
+		else
+			vmcs_write64(GUEST_BNDCFGS, vmx->nested.vmcs01_guest_bndcfgs);
+	}
 
 	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_IA32_PAT) {
 		vmcs_write64(GUEST_IA32_PAT, vmcs12->host_ia32_pat);
@@ -4466,6 +4470,10 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 		vmx_set_virtual_apic_mode(vcpu);
 	}
 
+	/* If not VM_EXIT_CLEAR_BNDCFGS, the L2 value propagates to L1.  */
+	if (!(vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS))
+		vmx->nested.vmcs01_guest_bndcfgs = vmcs12->guest_bndcfgs;
+
 	/* Unpin physical memory we referred to in vmcs02 */
 	if (vmx->nested.apic_access_page) {
 		kvm_release_page_clean(vmx->nested.apic_access_page);


which will also work in the failed vmentry case.

Paolo