linux-kernel - [RFC PATCH 18/21] KVM: x86: Invoke kvm_x86_ops->cpuid_update() after kvm_update

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Date:   Fri, 26 Jul 2019 22:52:11 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>
Subject: [RFC PATCH 18/21] KVM: x86: Invoke kvm_x86_ops->cpuid_update() after kvm_update_cpuid()

VMX's virtualization of SGX adds a lovely dependency on the guest's
supported xcr0, which is calculated in kvm_update_cpuid().  VMX must
toggled its interception of SGX instructions based on the supported
xcr0, i.e. kvm_x86_ops->cpuid_update() is certainly the correct
location for the dependent code.

kvm_update_cpuid() was originally added by commit 2acf923e38fb ("KVM:
VMX: Enable XSAVE/XRSTOR for guest").  There is no indication that its
placement after kvm_x86_ops->cpuid_update() was anything more than a
"new function at the end" decision.

Inspection of the current code reveals no dependency on kvm_x86_ops's
cpuid_update() in kvm_update_cpuid() or any of its helpers.

  - SVM's sole update is to conditionally clear X86_FEATURE_X2APIC.
    X86_FEATURE_X2APIC is only consumed by kvm_apic_set_state(), which
    is already called immediately prior to kvm_x86_ops->cpuid_update().

  - VMX updates only nested VMX MSRs, allowed FEATURE_CONTROL bits,
    and VMCS fields, e.g. secondary execution controls, none of which
    should bleed back into kvm_update_cpuid() barring an egregious
    dependency bug somewhere else.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/cpuid.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 70e488951f25..4c235af5318c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -222,8 +222,9 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 	vcpu->arch.cpuid_nent = cpuid->nent;
 	cpuid_fix_nx_cap(vcpu);
 	kvm_apic_set_version(vcpu);
-	kvm_x86_ops->cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (!r)
+		kvm_x86_ops->cpuid_update(vcpu);
 
 out:
 	vfree(cpuid_entries);
@@ -245,8 +246,9 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
 		goto out;
 	vcpu->arch.cpuid_nent = cpuid->nent;
 	kvm_apic_set_version(vcpu);
-	kvm_x86_ops->cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (!r)
+		kvm_x86_ops->cpuid_update(vcpu);
 out:
 	return r;
 }
-- 
2.22.0