linux-kernel - Re: [PATCH v9 11/22] KVM: x86: Add a helper to detect if FRED is enabled for a vCPU

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Message-ID: <aXE2BPCKvcIiQbqU@google.com>
Date: Wed, 21 Jan 2026 12:24:36 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin@...or.com>
Cc: Binbin Wu <binbin.wu@...ux.intel.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-doc@...r.kernel.org, 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, chao.gao@...el.com, 
	hch@...radead.org, sohil.mehta@...el.com
Subject: Re: [PATCH v9 11/22] KVM: x86: Add a helper to detect if FRED is
 enabled for a vCPU

On Wed, Jan 21, 2026, Xin Li wrote:
> 
> > On Jan 21, 2026, at 12:05 AM, Binbin Wu <binbin.wu@...ux.intel.com> wrote:
> > 
> > 
> > Not sure if it's OK with empty change log even though the patch is simple and
> > the title has already described it.
> 
> IIRC, Sean changed it this way ;)

I doubt that.  Ha!  Found it.  From: https://lore.kernel.org/all/ZmszIOsGtNsvqbpI@google.com
as an attachment:

[-- Attachment #4: 0013-KVM-x86-Add-a-helper-to-detect-if-FRED-is-enabled-fo.patch --]
[-- Type: text/x-diff, Size: 1418 bytes --]

>From f38dcc04e334cda572289f05f4be7702bebfc96a Mon Sep 17 00:00:00 2001
From: Xin Li <xin3.li@...el.com>
Date: Wed, 7 Feb 2024 09:26:31 -0800
Subject: [PATCH 13/28] KVM: x86: Add a helper to detect if FRED is enabled for
 a vCPU

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

Signed-off-by: Xin Li <xin3.li@...el.com>
Tested-by: Shan Kang <shan.kang@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.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 75eae9c4998a..fe5546efd388 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))
-- 
2.45.2.627.g7a2c4fd464-goog