linux-kernel - [PATCH v3 13/27] KVM: x86: Add a helper to detect if FRED is enabled for a vCPU

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Message-ID: <20241001050110.3643764-14-xin@zytor.com>
Date: Mon, 30 Sep 2024 22:00:56 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...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, luto@...nel.org, peterz@...radead.org,
        andrew.cooper3@...rix.com, xin@...or.com
Subject: [PATCH v3 13/27] KVM: x86: Add a helper to detect if FRED is enabled for a vCPU

From: Xin Li <xin3.li@...el.com>

Add is_fred_enabled() to detect if FRED is enabled on a vCPU.

Signed-off-by: Xin Li <xin3.li@...el.com>
[ Sean: removed the "kvm_" prefix from the function name ]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Xin Li (Intel) <xin@...or.com>
Tested-by: Shan Kang <shan.kang@...el.com>
---
 arch/x86/kvm/kvm_cache_regs.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index b1eb46e26b2e..386c79f5dcb8 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/arch/x86/kvm/kvm_cache_regs.h
@@ -187,6 +187,21 @@ 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.
+ */
+static inline bool 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))
-- 
2.46.2