Re: [PATCH 2/6] KVM: VMX: Passthrough MSR_IA32_PRED_CMD based purely on host+guest CPUID

Xiaoyao Li
2023-03-27T03:46:40Z

On 3/22/2023 9:14 AM, Sean Christopherson wrote:
> Passthrough MSR_IA32_PRED_CMD based purely on whether or not the MSR is
> supported and enabled, i.e. don't wait until the first write. There's no
> benefit to deferred passthrough, and the extra logic only adds complexity.

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 16 +++-------------
> 1 file changed, 3 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f777509ecf17..5c01c76c0d45 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2298,19 +2298,6 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> break;
>
> wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> -
> - /*
> - * For non-nested:
> - * When it's written (to non-zero) for the first time, pass
> - * it through.
> - *
> - * For nested:
> - * The handling of the MSR bitmap for L2 guests is done in
> - * nested_vmx_prepare_msr_bitmap. We should not touch the
> - * vmcs02.msr_bitmap here since it gets completely overwritten
> - * in the merging.
> - */
> - vmx_disable_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W);
> break;
> case MSR_IA32_CR_PAT:
> if (!kvm_pat_valid(data))
> @@ -7743,6 +7730,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R,
> !guest_cpuid_has(vcpu, X86_FEATURE_XFD));
>
> + if (boot_cpu_has(X86_FEATURE_IBPB))
> + vmx_set_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W,
> + !guest_has_pred_cmd_msr(vcpu));
>
> set_cr4_guest_host_mask(vmx);
>