linux-kernel - Re: [PATCH] KVM: x86/pt: Do not advertise Intel PT Event Trace capability

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Date:   Fri, 7 Jan 2022 19:18:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pt: Do not advertise Intel PT Event Trace
 capability

On 1/6/22 09:55, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> The Inte PT Event Trace capability (Intel SDM Vol3, 32.2.4 Event Tracing)
> is a new CPU feature that "exposes details about the asynchronous events,
> when they are generated, and when their corresponding software event
> handler completes execution".
> 
> It is not possible for KVM to emulate all events including interrupts,
> VM exits, VM entries, INIT, SIPI events and etc. for guests and to
> emulate the simultaneous writing of Control Flow Events and Event Data
> packets generated by the KVM to the guest PT buffer.
> 
> For KVM, it is best not to advertise the Event Trace feature and just
> let it be a system-wide-only tracing capability.
> 
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
> Off topic, other new PT features such as "PSB and PMI Preservation Supported"
> and "TNT disable" are under investigation or awaiting host support to move on.

Yeah, I think it's better to be safe and ignore _all_ unknown capabilities.

Paolo

>   arch/x86/kvm/cpuid.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..1028c57377e9 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -901,6 +901,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   			break;
>   		}
>   
> +		/* Not advertise Event Trace capability due to endless emulation */
> +		entry->ebx &= ~BIT(7);
>   		for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
>   			if (!do_host_cpuid(array, function, i))
>   				goto out;