linux-kernel - [PATCH 15/54] KVM: nSVM: Add a comment to document why nNPT uses vmcb01, not vCPU state

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Message-Id: <20210622175739.3610207-16-seanjc@google.com>
Date:   Tue, 22 Jun 2021 10:57:00 -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,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 15/54] KVM: nSVM: Add a comment to document why nNPT uses
 vmcb01, not vCPU state

Add a comment in the nested NPT initialization flow to call out that it
intentionally uses vmcb01 instead current vCPU state to get the effective
hCR4 and hEFER for L1's NPT context.

Note, despite nSVM's efforts to handle the case where vCPU state doesn't
reflect L1 state, the MMU may still do the wrong thing due to pulling
state from the vCPU instead of the passed in CR0/CR4/EFER values.  This
will be addressed in future commits.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/nested.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 9f0e7ed672b2..33b2f9337e26 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -98,6 +98,12 @@ static void nested_svm_init_mmu_context(struct kvm_vcpu *vcpu)
 	WARN_ON(mmu_is_nested(vcpu));
 
 	vcpu->arch.mmu = &vcpu->arch.guest_mmu;
+
+	/*
+	 * L1's CR4 and EFER are stuffed into vmcb01 by the caller.  Note, when
+	 * called via KVM_SET_NESTED_STATE, that state may _not_ match current
+	 * vCPU state.  CR0.WP is explicitly ignored, while CR0.PG is required.
+	 */
 	kvm_init_shadow_npt_mmu(vcpu, X86_CR0_PG, svm->vmcb01.ptr->save.cr4,
 				svm->vmcb01.ptr->save.efer,
 				svm->nested.ctl.nested_cr3);
-- 
2.32.0.288.g62a8d224e6-goog