[PATCH v4 13/19] KVM: x86: Mark CR4.FRED as not reserved

From: Xin Li (Intel)
Date: Fri Mar 28 2025 - 13:13:39 EST


From: Xin Li <xin3.li@xxxxxxxxx>

The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when
guest cpu cap has FRED, i.e.,
1) All of FRED KVM support is in place.
2) Guest enumerates FRED.

Otherwise it is still a reserved bit.

Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
---

Change in v4:
* Rebase on top of "guest_cpu_cap".

Change in v3:
* Don't allow CR4.FRED=1 before all of FRED KVM support is in place
(Sean Christopherson).
---
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/kvm/x86.h | 2 ++
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f8b9834f2f37..e94924397230 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -138,7 +138,7 @@
| X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
| X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
| X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
- | X86_CR4_LAM_SUP))
+ | X86_CR4_LAM_SUP | X86_CR4_FRED))

#define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 13dbd87970db..24661b2ad3ad 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -617,6 +617,8 @@ static inline bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
__reserved_bits |= X86_CR4_PCIDE; \
if (!__cpu_has(__c, X86_FEATURE_LAM)) \
__reserved_bits |= X86_CR4_LAM_SUP; \
+ if (!__cpu_has(__c, X86_FEATURE_FRED)) \
+ __reserved_bits |= X86_CR4_FRED; \
__reserved_bits; \
})

--
2.48.1