linux-kernel - Re: [PATCH] KVM: VMX: Update PT MSR intercepts during filter change iff PT in host+guest

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Message-ID: <34935bcd-0a5a-e239-d434-42187f94ad09@intel.com>
Date:   Tue, 12 Jul 2022 11:23:33 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Update PT MSR intercepts during filter change
 iff PT in host+guest

On 7/12/2022 9:58 AM, Sean Christopherson wrote:
> Update the Processor Trace (PT) MSR intercepts during a filter change if
> and only if PT may be exposed to the guest, i.e. only if KVM is operating
> in the so called "host+guest" mode where PT can be used simultaneously by
> both the host and guest.  If PT is in system mode, the host is the sole
> owner of PT and the MSRs should never be passed through to the guest.
> 
> Luckily the missed check only results in unnecessary work, as select RTIT
> MSRs are passed through only when RTIT tracing is enabled "in" the guest,
> and tracing can't be enabled in the guest when KVM is in system mode
> (writes to guest.MSR_IA32_RTIT_CTL are disallowed).

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 74ca64e97643..e6ab2c2c4d3b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4004,7 +4004,9 @@ static void vmx_msr_filter_changed(struct kvm_vcpu *vcpu)
>   			vmx_disable_intercept_for_msr(vcpu, msr, MSR_TYPE_W);
>   	}
>   
> -	pt_update_intercept_for_msr(vcpu);
> +	/* PT MSRs can be passed through iff PT is exposed to the guest. */
> +	if (vmx_pt_mode_is_host_guest())
> +		pt_update_intercept_for_msr(vcpu);
>   }
>   
>   static inline void kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
> 
> base-commit: 5406e590ac8fa33e390616031370806cdbcc5791