linux-kernel - Re: [PATCH] KVM: VMX: Disable Intel PT before VM-entry

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Date:   Thu, 9 Apr 2020 11:34:29 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Kang, Luwei" <luwei.kang@...el.com>
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>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>
Subject: Re: [PATCH] KVM: VMX: Disable Intel PT before VM-entry

On Mon, Mar 30, 2020 at 08:29:26PM -0700, Kang, Luwei wrote:
> > > > On Wed, Mar 18, 2020 at 11:48:18AM +0800, Luwei Kang wrote:
> > Ah, right.  What about enhancing intel_pt_handle_vmx() and 'struct pt' to
> > replace vmx_on with a field that incorporates the KVM mode?
> 
> Some history is the host perf didn't fully agree with introducing HOST_GUEST
> mode for PT in KVM. Because the KVM will disable the host trace before
> VM-entry in HOST_GUEST mode and KVM guest will win in this case. e.g. Intel
> PT has been enabled in KVM guest and the host wants to start system-wide
> trace(collect all the trace on this system) at this time, the trace produced
> by the Guest OS will be saved in guest PT buffer and host buffer can't get
> this. So I prefer don't introduce the KVM PT mode to host perf framework. The
> similar problem happens on PEBS virtualization via DS as well.

A maintainer's distaste for a feature isn't a good reason to put a hack
into KVM.  Perf burying its head in the sand won't change the fact that
"pt->vmx_on" is poorly named and misleading.  Disagreement over features
is fine, but things will go sideways quick if perf and KVM are outright
hostile towards each other.