Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to userspace

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Sat, Feb 24, 2018 at 01:52:26AM +0100, Paolo Bonzini wrote:
> Use the new MSR feature framework to expose the ARCH_CAPABILITIES MSR to
> userspace.  This way, userspace can access the capabilities even if it
> does not have the permissions to read MSRs.

... That is good but could you expand a bit of why it would want this?

I am 99% sure it is due to the lovely spectre_v2 mitigation but
could you include that in the commit message so that in say a year
folks would know what this is?

Also what branch is this based on? I am not seeing this vmx_get_msr_feature
in kvm/master or kvm/linux-next ?
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/vmx.c | 12 +++++++++++-
>  arch/x86/kvm/x86.c |  1 +
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 130fca0ea1bf..99689061e11e 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3228,7 +3228,17 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
>  
>  static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>  {
> -	return 1;
> +	switch (msr->index) {
> +	case MSR_IA32_ARCH_CAPABILITIES:
> +		if (!boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> +			return 1;
> +		rdmsrl(msr->index, msr->data);
> +		break;
> +	default:
> +		return 1;
> +	}
> +
> +	return 0;
>  }
>  
>  /*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 54b4ed55945b..e9a8cc9e3b2b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1054,6 +1054,7 @@ bool kvm_rdpmc(struct kvm_vcpu *vcpu)
>   * can be used by a hypervisor to validate requested CPU features.
>   */
>  static u32 msr_based_features[] = {
> +	MSR_IA32_ARCH_CAPABILITIES,
>  	MSR_F10H_DECFG,
>  };
>  
> -- 
> 1.8.3.1
>