linux-kernel - [PATCH 25/43] KVM: VMX: Pull GUEST_CR3 from the VMCS iff CR3 load exiting is disabled

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Message-Id: <20210424004645.3950558-26-seanjc@google.com>
Date:   Fri, 23 Apr 2021 17:46:27 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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: [PATCH 25/43] KVM: VMX: Pull GUEST_CR3 from the VMCS iff CR3 load
 exiting is disabled

Tweak the logic for grabbing vmcs.GUEST_CR3 in vmx_cache_reg() to look
directly at the execution controls, as opposed to effectively inferring
the controls from vCPUs state.  Inferring the controls isn't wrong, but it
creates a very subtle dependency between the caching logic, the state of
vcpu->arch.cr0 (via is_paging()), and the behavior of vmx_set_cr0().

Using the execution controls doesn't completely eliminate the dependency
in vmx_set_cr0(), e.g. neglecting to cache CR3 before enabling
interception would still break the guest, but it does reduce the
code dependency and mostly eliminate the logical dependency (that CR3
loads are intercepted in certain scenarios).  Eliminating the sublte
read of vcpu->arch.cr0 will also allow for additional cleanup in
vmx_set_cr0().

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e42ae77e4b82..596c8f9766ac 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2370,8 +2370,11 @@ static void vmx_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
 		vcpu->arch.cr0 |= vmcs_readl(GUEST_CR0) & guest_owned_bits;
 		break;
 	case VCPU_EXREG_CR3:
-		if (is_unrestricted_guest(vcpu) ||
-		    (enable_ept && is_paging(vcpu)))
+		/*
+		 * When intercepting CR3 loads, e.g. for shadowing paging, KVM's
+		 * CR3 is loaded into hardware, not the guest's CR3.
+		 */
+		if (!(exec_controls_get(to_vmx(vcpu)) & CPU_BASED_CR3_LOAD_EXITING))
 			vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
 		break;
 	case VCPU_EXREG_CR4:
-- 
2.31.1.498.g6c1eba8ee3d-goog