linux-kernel - Re: [PATCH 1/5] KVM: VMX: Restore host's MSR_IA32_RTIT

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Message-ID: <YSUycNbERUv6xGmB@google.com>
Date:   Tue, 24 Aug 2021 17:54:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's
 not zero

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@...el.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@...gle.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
>