linux-kernel - [PATCH 1/3] KVM: VMX: Allow userspace to set all supported FEATURE

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Message-Id: <20220607232353.3375324-2-seanjc@google.com>
Date:   Tue,  7 Jun 2022 23:23:51 +0000
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 1/3] KVM: VMX: Allow userspace to set all supported
 FEATURE_CONTROL bits

Allow userspace to set all supported bits in MSR IA32_FEATURE_CONTROL
irrespective of the guest CPUID model, e.g. via KVM_SET_MSRS.  KVM's ABI
is that userspace is allowed to set MSRs before CPUID, i.e. can set MSRs
to values that would fault according to the guest CPUID model.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fd2e707faf2b..8e83e12373c5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1741,12 +1741,38 @@ bool nested_vmx_allowed(struct kvm_vcpu *vcpu)
 	return nested && guest_cpuid_has(vcpu, X86_FEATURE_VMX);
 }
 
-static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
-						 uint64_t val)
+/*
+ * Userspace is allowed to set any supported IA32_FEATURE_CONTROL regardless of
+ * guest CPUID.  Note, KVM allows userspace to set "VMX in SMX" to maintain
+ * backwards compatibility even though KVM doesn't support emulating SMX.  And
+ * because userspace set "VMX in SMX", the guest must also be allowed to set it,
+ * e.g. if the MSR is left unlocked and the guest does a RMW operation.
+ */
+#define KVM_SUPPORTED_FEATURE_CONTROL  (FEAT_CTL_LOCKED			 | \
+					FEAT_CTL_VMX_ENABLED_INSIDE_SMX	 | \
+					FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX | \
+					FEAT_CTL_SGX_LC_ENABLED		 | \
+					FEAT_CTL_SGX_ENABLED		 | \
+					FEAT_CTL_LMCE_ENABLED)
+
+static inline bool vmx_feature_control_msr_valid(struct vcpu_vmx *vmx,
+						 struct msr_data *msr)
 {
-	uint64_t valid_bits = to_vmx(vcpu)->msr_ia32_feature_control_valid_bits;
+	uint64_t valid_bits;
 
-	return !(val & ~valid_bits);
+	/*
+	 * Ensure KVM_SUPPORTED_FEATURE_CONTROL is updated when new bits are
+	 * exposed to the guest.
+	 */
+	WARN_ON_ONCE(vmx->msr_ia32_feature_control_valid_bits &
+		     ~KVM_SUPPORTED_FEATURE_CONTROL);
+
+	if (msr->host_initiated)
+		valid_bits = KVM_SUPPORTED_FEATURE_CONTROL;
+	else
+		valid_bits = vmx->msr_ia32_feature_control_valid_bits;
+
+	return !(msr->data & ~valid_bits);
 }
 
 static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
@@ -2139,7 +2165,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vcpu->arch.mcg_ext_ctl = data;
 		break;
 	case MSR_IA32_FEAT_CTL:
-		if (!vmx_feature_control_msr_valid(vcpu, data) ||
+		if (!vmx_feature_control_msr_valid(vmx, msr_info) ||
 		    (to_vmx(vcpu)->msr_ia32_feature_control &
 		     FEAT_CTL_LOCKED && !msr_info->host_initiated))
 			return 1;
-- 
2.36.1.255.ge46751e96f-goog