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

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Date:   Fri, 20 Oct 2017 00:22:01 +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 19/10/2017 07:54, Kang, Luwei wrote:
> >>> Get it. I have feedback to hardware architect. I hope it can be applied but it may need wait a long time.
> >> Note that the hardware need not do anything.  However it would be
> >> nice if the SDM can define a bit _for the hypervisors_ to enforce the above constraint and fail vmentry if they are not respected.
> >
> > Hi Paolo,
> > Thanks for your response. I have a question want to ask for you. As
> > you mentioned in previous mail " We would like the nested hypervisor
> > to be forced to set the "use GPA for processor tracing"". Is there
> > have any problem if we don't set "use GPA for processor tracing" in
> > nested hypervisor?
> 
> If the nested hypervisor doesn't set "use GPA for processor tracing", the processor should use L1 addresses for processor tracing.
> This however is not possible without shadowing the ToPA, same as in non-nested virtualization for <=Skylake processors.
> 
> So we have
> 
>             |          mode
>             |-----------------------------------------------------------
> nested?     |          system-wide          |  host-guest
> ------------+-------------------------------+---------------------------
> no          |          use HPA for tracing  |  use GPA for tracing
>             |          (no EPT)             |  (EPT is GPA->HPA)
> ------------+-------------------------------+---------------------------
> yes         |          use GPA for tracing  |  use nGPA for tracing
>             |          (EPT is nGPA->HPA!!) |  (EPT is nGPA->HPA, so ok)
> 
> (for nested, L0 mode of course must be host-guest).  If the nested hypervisor wants to use system-wide tracing, it cannot use "use
> GPA for tracing" because the EPT table doesn't have the right mapping of L1->L0 physical address.
> 
> So if you want to do system-wide L1 tracing you have to disable EPT for L1, and if you want to do host-guest L1 tracing you have to
> enable it.
> 
HI Paolo,
    Thanks for your clarify. Have understood. So, we should set "use GPA for processor tracing" in any way( if we can do it) even in system mode. There don't have problem in no nested but have problem in nested if not set this bit.
    Still talking with  hardware designer but please don't expect it can be change in SDM or hardware(fail vmentry if they are not respected) soon. So, can we enable it in L1 guest only first?  I think it is not worth to disable EPT for L1 to enable intel PT. what is your opinion?

Thanks,
Luwei Kang