linux-kernel - [PATCH v2 11/25] KVM: x86: Add kvm_is_fred

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Date: Wed,  7 Feb 2024 09:26:31 -0800
From: Xin Li <xin3.li@...el.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Cc: seanjc@...gle.com,
	pbonzini@...hat.com,
	corbet@....net,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	x86@...nel.org,
	hpa@...or.com,
	shuah@...nel.org,
	vkuznets@...hat.com,
	peterz@...radead.org,
	ravi.v.shankar@...el.com,
	xin@...or.com
Subject: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred_enabled()

Add kvm_is_fred_enabled() to get if FRED is enabled on a vCPU.

Signed-off-by: Xin Li <xin3.li@...el.com>
Tested-by: Shan Kang <shan.kang@...el.com>
---

Change since v1:
* Explain why it is ok to only check CR4.FRED (Chao Gao).
---
 arch/x86/kvm/kvm_cache_regs.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index 75eae9c4998a..1d431c703fdf 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/arch/x86/kvm/kvm_cache_regs.h
@@ -187,6 +187,23 @@ static __always_inline bool kvm_is_cr4_bit_set(struct kvm_vcpu *vcpu,
 	return !!kvm_read_cr4_bits(vcpu, cr4_bit);
 }
 
+/*
+ * It's enough to check just CR4.FRED (X86_CR4_FRED) to tell if
+ * a vCPU is running with FRED enabled, because:
+ * 1) CR4.FRED can be set to 1 only _after_ IA32_EFER.LMA = 1.
+ * 2) To leave IA-32e mode, CR4.FRED must be cleared first.
+ *
+ * More details at FRED Spec 6.0 Section 4.2 Enabling in CR4.
+ */
+static __always_inline bool kvm_is_fred_enabled(struct kvm_vcpu *vcpu)
+{
+#ifdef CONFIG_X86_64
+	return kvm_is_cr4_bit_set(vcpu, X86_CR4_FRED);
+#else
+	return false;
+#endif
+}
+
 static inline ulong kvm_read_cr3(struct kvm_vcpu *vcpu)
 {
 	if (!kvm_register_is_available(vcpu, VCPU_EXREG_CR3))
-- 
2.43.0