On Sun, Oct 26, 2025 at 01:19:03PM -0700, Xin Li (Intel) wrote:
>From: Xin Li <xin3.li@intel.com>
>
>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>
I am not sure about two things regarding CR4.FRED and emulator code:
1. Should kvm_set_cr4() reject setting CR4.FRED when the vCPU isn't in long
mode? The concern is that emulator code may call kvm_set_cr4(). This could
cause VM-entry failure if CR4.FRED is set in other modes.
2. mk_cr_64() drops the high 32 bits of the new CR4 value. So, CR4.FRED is always
dropped. This may need an update.
This patch itself looks good, so:
Reviewed-by: Chao Gao <chao.gao@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
>