linux-kernel - RE: [PATCH v1 3/8] KVM: x86: Allocate performance counter for PEBS event

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Message-ID: <82D7661F83C1A047AF7DC287873BF1E17383642D@SHSMSX104.ccr.corp.intel.com>
Date:   Thu, 31 Oct 2019 11:10:18 +0000
From:   "Kang, Luwei" <luwei.kang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
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Subject: RE: [PATCH v1 3/8] KVM: x86: Allocate performance counter for PEBS
 event

> > > >  static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
> > > >  				  unsigned config, bool exclude_user,
> > > >  				  bool exclude_kernel, bool intr,
> > > > -				  bool in_tx, bool in_tx_cp)
> > > > +				  bool in_tx, bool in_tx_cp, bool pebs)
> > > >  {
> > > >  	struct perf_event *event;
> > > >  	struct perf_event_attr attr = {
> > > > @@ -111,9 +111,12 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc,
> u32 type,
> > > >  		.exclude_user = exclude_user,
> > > >  		.exclude_kernel = exclude_kernel,
> > > >  		.config = config,
> > > > +		.precise_ip = pebs ? 1 : 0,
> > > > +		.aux_output = pebs ? 1 : 0,
> > >
> > > srsly?
> >
> > Hi Peter,
> >     Thanks for review. For aux_output, I think it should be set 1 when the guest wants to
> enabled PEBS by Intel PT.
> >      For precise_ip, it is the precise level in perf and set by perf command line in KVM
> guest, this may not reflect the accurate value (can be 0~3) here. Here set to 1 is used to
> allocate a counter for PEBS event and set the MSR_IA32_PEBS_ENABLE register. For
> PMU virtualization, KVM will trap the guest's write operation to PMU registers and
> allocate/free event counter from host if a counter enable/disable in guest. We can't
> always deduce the exact parameter of perf command line from the value of the guest
> writers to the register.
> 
> Please, teach your MUA to wrap on 78 chars.
> 
> The thing I really fell over is the gratuitous 'bool ? 1 : 0'. But yes, the aux_output without
> a group leader is dead wrong. We'll go fix
> perf_event_create_kernel_counter() to refuse that.

Yes, I also think it is a little gratuitous here. But it is a little hard to reconstruct the guest perf parameters from the register value, especially the "precise_ip". Do you have any advice?

About refuse the perf_event_create_kernel_counter() request w/o aux_ouput, I think I need to allocate the PT event as group leader here,  is that right?

Thanks,
Luwei Kang