[PATCH v8 14/21] KVM: x86: Mark CR4.FRED as not reserved

Xin Li (Intel)
2025-10-14T01:11:13Z

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@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
---

Change in v5:
* Add TB from Xuelian Guo.

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 5fff22d837aa..558f260a1afd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -142,7 +142,7 @@
| 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_CET))
+ | X86_CR4_LAM_SUP | X86_CR4_CET | 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 4f5d12d7136e..e9c6f304b02e 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -687,6 +687,8 @@ static inline bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
if (!__cpu_has(__c, X86_FEATURE_SHSTK) && \
!__cpu_has(__c, X86_FEATURE_IBT)) \
__reserved_bits |= X86_CR4_CET; \
+ if (!__cpu_has(__c, X86_FEATURE_FRED)) \
+ __reserved_bits |= X86_CR4_FRED; \
__reserved_bits; \
})

--
2.51.0