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

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Message-ID: <0ca0d53b-640a-c124-08dc-dbf009e01a8e@redhat.com>
Date:   Wed, 18 Oct 2017 12:30:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Kang, Luwei" <luwei.kang@...el.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 18/10/2017 12:18, Kang, Luwei wrote:
>> 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.

Well, it's required more than recommended.  However, it's only required
if "enable EPT" is set and RTIT_CTL is nonzero.

> 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.

Correct.  The constraint would be:

* RTIT_CTL on entry is zero if EPT is disabled

* RTIT_CTL on entry is zero if EPT is enabled and "Guest PT uses GPA" is
zero

Maybe IA32_VMX_EPT_VPID_CAP is not the best place.  I'll let Intel
decide that.

Paolo