Re: [PATCH v3 13/21] KVM:VMX: Emulate reads and writes to CET MSRs

[Binbin Wu]

On 5/11/2023 12:08 PM, Yang Weijiang wrote:
> Add support for emulating read and write accesses to CET MSRs.
> CET MSRs are universally "special" as they are either context switched
> via dedicated VMCS fields or via XSAVES, i.e. no additional in-memory
> tracking is needed, but emulated reads/writes are more expensive.
>
> Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>   arch/x86/kernel/fpu/core.c |  1 +
>   arch/x86/kvm/vmx/vmx.c     | 18 ++++++++++++++++++
>   arch/x86/kvm/x86.c         | 20 ++++++++++++++++++++
>   arch/x86/kvm/x86.h         | 31 +++++++++++++++++++++++++++++++
>   4 files changed, 70 insertions(+)
...
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b6eec9143129..2e3a39c9297c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13630,6 +13630,26 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>   }
>   EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>   
> +bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu, struct msr_data *msr)
> +{
> +	if (!kvm_cet_user_supported())
> +		return false;
> +
> +	if (msr->host_initiated)
> +		return true;
> +
> +	if (!guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) &&
> +	    !guest_cpuid_has(vcpu, X86_FEATURE_IBT))
> +		return false;
> +
> +	if (msr->index == MSR_IA32_PL3_SSP &&
> +	    !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
> +		return false;
It may be better to merge the two if statements into one to avoid 
calling guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) twice.

e.g,

    if (!guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) &&
        (!guest_cpuid_has(vcpu, X86_FEATURE_IBT) || msr->index == 
MSR_IA32_PL3_SSP))
        return false;


> +
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(kvm_cet_is_msr_accessible);
> +
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
...