linux-kernel - RE: [PATCH v7 05/13] perf/x86/intel/pt: Introduce a new function to get capability of Intel PT

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Date:   Thu, 3 May 2018 12:50:48 +0000
From:   "Kang, Luwei" <luwei.kang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>
Subject: RE: [PATCH v7 05/13] perf/x86/intel/pt: Introduce a new function to
 get capability of Intel PT

> >> Can you elaborate on this, what information do we need besides
> >> MSR_IA32_VMX_MISC[14]?
> >
> > Enable Intel PT in guest depend on SECONDARY_EXEC_PT_USE_GPA, EPT,
> > MSR_IA32_VMX_MISC[14] and the capability of " PT_CAP_topa_output" on
> > hardware. PT driver will return " -ENODEV" if hardware not support
> > "PT_CAP_topa_output".
> 
> I actually don't understand why PT_CAP_topa_output matters for the purpose of enabling PT in the guest.  However you still need
> __pt_cap_get() in the CPUID checks.

Hi Paolo,
I think we should expose Intel Processor Trace to guest that can be detected and initialized. But without "PT_CAP_topa_output" Intel PT can't work in Linux.  So I add this feature as precondition. 
About why need this check in driver I think Alexander may know the reason.

Thanks,
Luwei Kang

> 
> Paolo