Re: [PATCH 11/15] KVM: VMX: Disable loading of TSX_CTRL MSR the more conventional way

[Maxim Levitsky]

On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote:
> Tag TSX_CTRL as not needing to be loaded when RTM isn't supported in the
> host.  Crushing the write mask to '0' has the same effect, but requires
> more mental gymnastics to understand.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4b432d2bbd06..7a53568b34fc 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1771,7 +1771,13 @@ static void setup_msrs(struct vcpu_vmx *vmx)
>  			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP) ||
>  			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDPID));
>  
> -	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, true);
> +	/*
> +	 * hle=0, rtm=0, tsx_ctrl=1 can be found with some combinations of new
> +	 * kernel and old userspace.  If those guests run on a tsx=off host, do
> +	 * allow guests to use TSX_CTRL, but don't change the value in hardware
> +	 * so that TSX remains always disabled.
> +	 */
> +	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, boot_cpu_has(X86_FEATURE_RTM));
>  
>  	if (cpu_has_vmx_msr_bitmap())
>  		vmx_update_msr_bitmap(&vmx->vcpu);
> @@ -6919,23 +6925,15 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
>  		vmx->guest_uret_msrs[i].data = 0;
>  		vmx->guest_uret_msrs[i].mask = -1ull;
>  	}
> -	tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
> -	if (tsx_ctrl) {
> +	if (boot_cpu_has(X86_FEATURE_RTM)) {
>  		/*
>  		 * TSX_CTRL_CPUID_CLEAR is handled in the CPUID interception.
>  		 * Keep the host value unchanged to avoid changing CPUID bits
>  		 * under the host kernel's feet.
> -		 *
> -		 * hle=0, rtm=0, tsx_ctrl=1 can be found with some combinations
> -		 * of new kernel and old userspace.  If those guests run on a
> -		 * tsx=off host, do allow guests to use TSX_CTRL, but do not
> -		 * change the value on the host so that TSX remains always
> -		 * disabled.
>  		 */
> -		if (boot_cpu_has(X86_FEATURE_RTM))
> +		tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
> +		if (tsx_ctrl)
>  			vmx->guest_uret_msrs[i].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
> -		else
> -			vmx->guest_uret_msrs[i].mask = 0;
>  	}
>  
>  	err = alloc_loaded_vmcs(&vmx->vmcs01);

I also agree that commit message should be updated as Paolo said,
but other than that:

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky <mlevitsk@xxxxxxxxxx>