linux-kernel - [RFC PATCH v3 23/59] KVM: x86: Allow host-initiated WRMSR to set X2APIC regardless of CPUID

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Date:   Wed, 24 Nov 2021 16:20:06 -0800
From:   isaku.yamahata@...el.com
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     isaku.yamahata@...el.com, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: [RFC PATCH v3 23/59] KVM: x86: Allow host-initiated WRMSR to set X2APIC regardless of CPUID

From: Sean Christopherson <sean.j.christopherson@...el.com>

Let userspace, or in the case of TDX, KVM itself, enable X2APIC even if
X2APIC is not reported as supported in the guest's CPU model.  KVM
generally does not force specific ordering between ioctls(), e.g. this
forces userspace to configure CPUID before MSRs.  And for TDX, vCPUs
will always run with X2APIC enabled, e.g. KVM will want/need to enable
X2APIC from time zero.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
---
 arch/x86/kvm/x86.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f2b6a3f89e9e..0221ef691a15 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -466,8 +466,11 @@ int kvm_set_apic_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 {
 	enum lapic_mode old_mode = kvm_get_apic_mode(vcpu);
 	enum lapic_mode new_mode = kvm_apic_mode(msr_info->data);
-	u64 reserved_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu) | 0x2ff |
-		(guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) ? 0 : X2APIC_ENABLE);
+	u64 reserved_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu) | 0x2ff;
+
+	if (!msr_info->host_initiated &&
+	    !guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
+		reserved_bits |= X2APIC_ENABLE;
 
 	if ((msr_info->data & reserved_bits) != 0 || new_mode == LAPIC_MODE_INVALID)
 		return 1;
-- 
2.25.1