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

[Paolo Bonzini]

On 04/05/21 19:17, 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.

This doesn't explain _why_ this is now possible.  What about:

Now that user return MSRs is always present in the list, we don't have
the problem that the TSX_CTRL MSR needs a slot vmx->guest_uret_msrs even
if RTM is not supported in the host (and therefore there is nothing to
enable).  Thus we can simply tag TSX_CTRL as not needing to be loaded
instead of crushing the write mask to '0'.  That has the same effect,
but requires less mental gymnastics to understand.

Paolo

> 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);