Re: [PATCH v3 17/27] KVM: x86: Mark CR4.FRED as not reserved when guest can use FRED

[Xin Li]

On 10/24/2024 12:18 AM, Chao Gao wrote:
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 03f42b218554..bfdd10773136 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -8009,6 +8009,10 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
> > 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_FRED);
> >
> > +	/* Don't allow CR4.FRED=1 before all of FRED KVM support is in place. */
> > +	if (!guest_can_use(vcpu, X86_FEATURE_FRED))
> > +		vcpu->arch.cr4_guest_rsvd_bits |= X86_CR4_FRED;
>
> is this necessary? __kvm_is_valid_cr4() ensures that guests cannot set any bit
> which isn't supported by the hardware.
>
> To account for hardware/KVM caps, I think the following changes will work. This
> will fix all other bits besides X86_CR4_FRED.

This seems a generic infra improvement, maybe it's better for you to
send it as an individual patch to Sean and the KVM mailing list?

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4a93ac1b9be9..2bec3ba8e47d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1873,6 +1873,7 @@ struct kvm_arch_async_pf {
>   extern u32 __read_mostly kvm_nr_uret_msrs;
>   extern bool __read_mostly allow_smaller_maxphyaddr;
>   extern bool __read_mostly enable_apicv;
> +extern u64 __read_mostly cr4_reserved_bits;
>   extern struct kvm_x86_ops kvm_x86_ops;
>   
>   #define kvm_x86_call(func) static_call(kvm_x86_##func)
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2617be544480..57d82fbcfd3f 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -393,8 +393,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>   	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
>   
>   	kvm_pmu_refresh(vcpu);
> -	vcpu->arch.cr4_guest_rsvd_bits =
> -	    __cr4_reserved_bits(guest_cpuid_has, vcpu);
> +	vcpu->arch.cr4_guest_rsvd_bits = cr4_reserved_bits |
> +					 __cr4_reserved_bits(guest_cpuid_has, vcpu);
>   
>   	kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu->arch.cpuid_entries,
>   						    vcpu->arch.cpuid_nent));
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 34b52b49f5e6..08b42bbd2342 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -119,7 +119,7 @@ u64 __read_mostly efer_reserved_bits = ~((u64)(EFER_SCE | EFER_LME | EFER_LMA));
>   static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
>   #endif
>   
> -static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
> +u64 __read_mostly cr4_reserved_bits;
>   
>   #define KVM_EXIT_HYPERCALL_VALID_MASK (1 << KVM_HC_MAP_GPA_RANGE)
>   
> @@ -1110,13 +1110,7 @@ EXPORT_SYMBOL_GPL(kvm_emulate_xsetbv);
>   
>   bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>   {
> -	if (cr4 & cr4_reserved_bits)
> -		return false;
> -
> -	if (cr4 & vcpu->arch.cr4_guest_rsvd_bits)
> -		return false;
> -
> -	return true;
> +	return !(cr4 & vcpu->arch.cr4_guest_rsvd_bits);
>   }
>   EXPORT_SYMBOL_GPL(__kvm_is_valid_cr4);
>   
>
> > +
> > 	vmx_setup_uret_msrs(vmx);
> >
> > 	if (cpu_has_secondary_exec_ctrls())
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 992e73ee2ec5..0ed91512b757 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -561,6 +561,8 @@ enum kvm_msr_access {
> > 		__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;                                \
> > })
> >
> > --
> > 2.46.2