linux-kernel - Re: [PATCH v11 08/11] KVM: x86/pmu: Emulate LBR feature via guest LBR event

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Date:   Tue, 19 May 2020 20:24:38 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>, ak@...ux.intel.com,
        wei.w.wang@...el.com
Subject: Re: [PATCH v11 08/11] KVM: x86/pmu: Emulate LBR feature via guest LBR
 event

On 2020/5/19 19:00, Peter Zijlstra wrote:
> On Thu, May 14, 2020 at 04:30:51PM +0800, Like Xu wrote:
>> +static inline bool event_is_oncpu(struct perf_event *event)
>> +{
>> +	return event && event->oncpu != -1;
>> +}
>
>> +/*
>> + * It's safe to access LBR msrs from guest when they have not
>> + * been passthrough since the host would help restore or reset
>> + * the LBR msrs records when the guest LBR event is scheduled in.
>> + */
>> +static bool intel_pmu_access_lbr_msr(struct kvm_vcpu *vcpu,
>> +				     struct msr_data *msr_info, bool read)
>> +{
>> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> +	u32 index = msr_info->index;
>> +
>> +	if (!intel_is_valid_lbr_msr(vcpu, index))
>> +		return false;
>> +
>> +	if (!msr_info->host_initiated && !pmu->lbr_event)
>> +		intel_pmu_create_lbr_event(vcpu);
>> +
>> +	/*
>> +	 * Disable irq to ensure the LBR feature doesn't get reclaimed by the
>> +	 * host at the time the value is read from the msr, and this avoids the
>> +	 * host LBR value to be leaked to the guest. If LBR has been reclaimed,
>> +	 * return 0 on guest reads.
>> +	 */
>> +	local_irq_disable();
>> +	if (event_is_oncpu(pmu->lbr_event)) {
>> +		if (read)
>> +			rdmsrl(index, msr_info->data);
>> +		else
>> +			wrmsrl(index, msr_info->data);
>> +	} else if (read)
>> +		msr_info->data = 0;
>> +	local_irq_enable();
>> +
>> +	return true;
>> +}
> So this runs in the vCPU thread in host context to emulate the MSR
> access, right?
Yes, it's called to emulate MSR accesses when the guest LBR event is
scheduled on while the LBR stack MSRs have not been passthrough to the vCPU.

Thanks,
Like Xu
>