Re: [PATCH 1/5] KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's not zero

From: Sean Christopherson <seanjc@google.com>
To: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/5] KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's not zero
Date: Tue, 24 Aug 2021 17:54:56 +0000	[thread overview]
Message-ID: <YSUycNbERUv6xGmB@google.com> (raw)
In-Reply-To: <20210824110743.531127-2-xiaoyao.li@intel.com>

On Tue, Aug 24, 2021, Xiaoyao Li wrote:
> A minor optimation to WRMSR MSR_IA32_RTIT_CTL when necessary.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fada1055f325..e0a9460e4dab 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1075,7 +1075,8 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
>  	}
>  
>  	/* Reload host state (IA32_RTIT_CTL will be cleared on VM exit). */

Could you opportunistically update the comment to call out that KVM requires
VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest?  E.g. something like

	/*
	 * KVM's requires VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest,
	 * i.e. RTIT_CTL is always cleared on VM-Exit.  Restore it if necessary.
	 */

With that,

Reviewed-by: Sean Christopherson <seanjc@google.com> 

> -	wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
> +	if (vmx->pt_desc.host.ctl)
> +		wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
>  }
>  
>  void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
> -- 
> 2.27.0
> 