Re: [Patch v3] KVM: x86/pmu: Manipulate FIXED_CTR_CTRL MSR with macros

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 3/6/2024 7:22 AM, Sean Christopherson wrote:
> +Mingwei
>
> On Thu, Aug 24, 2023, Dapeng Mi wrote:
>   diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
>> index 7d9ba301c090..ffda2ecc3a22 100644
>> --- a/arch/x86/kvm/pmu.h
>> +++ b/arch/x86/kvm/pmu.h
>> @@ -12,7 +12,8 @@
>>   					  MSR_IA32_MISC_ENABLE_BTS_UNAVAIL)
>>   
>>   /* retrieve the 4 bits for EN and PMI out of IA32_FIXED_CTR_CTRL */
>> -#define fixed_ctrl_field(ctrl_reg, idx) (((ctrl_reg) >> ((idx)*4)) & 0xf)
>> +#define fixed_ctrl_field(ctrl_reg, idx) \
>> +	(((ctrl_reg) >> ((idx) * INTEL_FIXED_BITS_STRIDE)) & INTEL_FIXED_BITS_MASK)
>>   
>>   #define VMWARE_BACKDOOR_PMC_HOST_TSC		0x10000
>>   #define VMWARE_BACKDOOR_PMC_REAL_TIME		0x10001
>> @@ -165,7 +166,8 @@ static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
>>   
>>   	if (pmc_is_fixed(pmc))
>>   		return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
>> -					pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
>> +					pmc->idx - INTEL_PMC_IDX_FIXED) &
>> +					(INTEL_FIXED_0_KERNEL | INTEL_FIXED_0_USER);
>>   
>>   	return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
>>   }
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index f2efa0bf7ae8..b0ac55891cb7 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -548,8 +548,13 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>>   		setup_fixed_pmc_eventsel(pmu);
>>   	}
>>   
>> -	for (i = 0; i < pmu->nr_arch_fixed_counters; i++)
>> -		pmu->fixed_ctr_ctrl_mask &= ~(0xbull << (i * 4));
>> +	for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
>> +		pmu->fixed_ctr_ctrl_mask &=
>> +			 ~intel_fixed_bits_by_idx(i,
>> +						  INTEL_FIXED_0_KERNEL |
>> +						  INTEL_FIXED_0_USER |
>> +						  INTEL_FIXED_0_ENABLE_PMI);
>> +	}
>>   	counter_mask = ~(((1ull << pmu->nr_arch_gp_counters) - 1) |
>>   		(((1ull << pmu->nr_arch_fixed_counters) - 1) << INTEL_PMC_IDX_FIXED));
>>   	pmu->global_ctrl_mask = counter_mask;
>> @@ -595,7 +600,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>>   			pmu->reserved_bits &= ~ICL_EVENTSEL_ADAPTIVE;
>>   			for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
>>   				pmu->fixed_ctr_ctrl_mask &=
>> -					~(1ULL << (INTEL_PMC_IDX_FIXED + i * 4));
> OMG, this might just win the award for most obfuscated PMU code in KVM, which is
> saying something.  The fact that INTEL_PMC_IDX_FIXED happens to be 32, the same
> bit number as ICL_FIXED_0_ADAPTIVE, is 100% coincidence.  Good riddance.
>
> Argh, and this goofy code helped introduce a real bug.  reprogram_fixed_counters()
> doesn't account for the upper 32 bits of IA32_FIXED_CTR_CTRL.
>
> Wait, WTF?  Nothing in KVM accounts for the upper bits.  This can't possibly work.
>
> IIUC, because KVM _always_ sets precise_ip to a non-zero bit for PEBS events,
> perf will _always_ generate an adaptive record, even if the guest requested a
> basic record.  Ugh, and KVM will always generate adaptive records even if the
> guest doesn't support them.  This is all completely broken.  It probably kinda
> sorta works because the Basic info is always stored in the record, and generating
> more info requires a non-zero MSR_PEBS_DATA_CFG, but ugh.
>
> Oh great, and it gets worse.  intel_pmu_disable_fixed() doesn't clear the upper
> bits either, i.e. leaves ICL_FIXED_0_ADAPTIVE set.  Unless I'm misreading the code,
> intel_pmu_enable_fixed() effectively doesn't clear ICL_FIXED_0_ADAPTIVE either,
> as it only modifies the bit when it wants to set ICL_FIXED_0_ADAPTIVE.


Currently the host PMU driver would always set the "Adaptive_Record" bit 
in PERFEVTSELx and FIXED_CNTR_CTR MSRs as long as HW supports the 
adaptive PEBS feature (See details in helpers intel_pmu_pebs_enable() 
and intel_pmu_enable_fixed()).

It looks perf system doesn't export a interface to enable/disable the 
adaptive PEBS.  I suppose that's why KVM doesn't handle the 
"Adaptive_Record" bit in ERFEVTSELx and FIXED_CNTR_CTR MSRs.


>
> *sigh*
>
> I'm _very_ tempted to disable KVM PEBS support for the current PMU, and make it
> available only when the so-called passthrough PMU is available[*].  Because I
> don't see how this is can possibly be functionally correct, nor do I see a way
> to make it functionally correct without a rather large and invasive series.
>
> Ouch.  And after chatting with Mingwei, who asked the very good question of
> "can this leak host state?", I am pretty sure that yes, this can leak host state.
>
> When PERF_CAP_PEBS_BASELINE is enabled for the guest, i.e. when the guest has
> access to adaptive records, KVM gives the guest full access to MSR_PEBS_DATA_CFG
>
> 	pmu->pebs_data_cfg_mask = ~0xff00000full;
>
> which makes sense in a vacuum, because AFAICT the architecture doesn't allow
> exposing a subset of the four adaptive controls.
>
> GPRs and XMMs are always context switched and thus benign, but IIUC, Memory Info
> provides data that might now otherwise be available to the guest, e.g. if host
> userspace has disallowed equivalent events via KVM_SET_PMU_EVENT_FILTER.
>
> And unless I'm missing something, LBRs are a full leak of host state.  Nothing
> in the SDM suggests that PEBS records honor MSR intercepts, so unless KVM is
> also passing through LBRs, i.e. is context switching all LBR MSRs, the guest can
> use PEBS to read host LBRs at will.

Not sure If I missed something, but I don't see there is leak of host 
state. All perf events created by KVM would set "exclude_host" 
attribute, that would leads to all guest counters including the PEBS 
enabling counters would be disabled immediately by VMX once vm exits, 
and so the PEBS engine would stop as well. I don't see a PEBS record 
contains host state is possible to be written into guest DS area.


>
> Unless someone chimes in to point out how PEBS virtualization isn't a broken mess,
> I will post a patch to effectively disable PEBS virtualization.
>
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index 41a4533f9989..a2f827fa0ca1 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -392,7 +392,7 @@ static inline bool vmx_pt_mode_is_host_guest(void)
>   
>   static inline bool vmx_pebs_supported(void)
>   {
> -       return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
> +       return false;
>   }
>   
>   static inline bool cpu_has_notify_vmexit(void)