linux-kernel - RE: [PATCH 0/9] Intel Processor Trace virtulization enabling

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Date:   Wed, 18 Oct 2017 10:18:39 +0000
From:   "Kang, Luwei" <luwei.kang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chao Peng <chao.p.peng@...ux.intel.com>
Subject: RE: [PATCH 0/9] Intel Processor Trace virtulization enabling

> On 16/10/2017 14:09, Luwei Kang wrote:
> > 2. Enabling use of EPT to redirect PT output.
> >   — This enables the VMM to elect to virtualize the PT output buffer using EPT. In this mode, the CPU will treat PT output
> addresses as Guest Physical Addresses (GPAs) and translate them using EPT. This means that Intel PT output reads (of the ToPA
> table) and writes (of trace output) can cause EPT violations, and other output events.
> 
> Yay yay yay! ;)
> 
> > Intel Processor Trace virtualization can be work in one of 4 possible modes by set new option "pt_mode". Default is host guest
> mode.
> >  a. system-wide: trace both host/guest and output to host buffer;  b.
> > host-only: only trace host and output to host buffer;  c. guest-only:
> > only trace guest and output to guest buffer;  d. host-guest: trace
> > host/guest simultaneous and output to their respective buffer.
> 
> I think the guest-only mode isn't particularly useful, and I would remove it.

Agree, will remove it.

> 
> Nested virtualization is interesting.  We would like the nested hypervisor to be forced to set the "use GPA for processor tracing"
> secondary execution control whenever "enable EPT" is set and RTIT_CTL is nonzero.  There is no way to encode that in
> IA32_VMX_PROCBASED_CTLS2, however.  It would be nice if Intel could reserve a bit in IA32_VMX_EPT_VPID_CAP for KVM to
> express that constraint.

Do you mean if nested hypervisor get the capability of "Guest PT use GPA" and EPT has enable. Highly recommend nested hypervisor set " Guest PT use GPA "  as well. 
If nested hypervisor is also KVM, "use GPA for processor tracing" will be set for sure. But other hypervisor may not do that. So, we'd better add a flag in IA32_VMX_EPT_VPID_CAP to express that constraint.  Is that right?

Thanks,
Luwei Kang