linux-kernel - Re: [PATCH 2/3] KVM: x86: Register Processor Trace interrupt hook iff PT enabled in guest

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Date:   Wed, 25 Aug 2021 20:09:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Artem Kashkanov <artem.kashkanov@...el.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/3] KVM: x86: Register Processor Trace interrupt hook
 iff PT enabled in guest

On Wed, Aug 25, 2021, Sean Christopherson wrote:
> On Wed, Aug 25, 2021, Like Xu wrote:
> > On 24/8/2021 3:37 am, Sean Christopherson wrote:
> > > @@ -11061,6 +11061,8 @@ int kvm_arch_hardware_setup(void *opaque)
> > >   	memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
> > >   	kvm_ops_static_call_update();
> > > +	if (ops->intel_pt_intr_in_guest && ops->intel_pt_intr_in_guest())
> > > +		kvm_guest_cbs.handle_intel_pt_intr = kvm_handle_intel_pt_intr;
> > 
> > Emm, it's still buggy.
> > 
> > The guest "unknown NMI" from the host Intel PT can still be reproduced
> > after the following operation:
> > 
> > 	rmmod kvm_intel
> > 	modprobe kvm-intel pt_mode=1 ept=1
> > 	rmmod kvm_intel
> > 	modprobe kvm-intel pt_mode=1 ept=0
> > 
> > Since the handle_intel_pt_intr is not reset to NULL in kvm_arch_hardware_unsetup(),
> > and the previous function pointer still exists in the generic KVM data structure.
> 
> Ooof, good catch.  Any preference between nullifying handle_intel_pt_intr in
> setup() vs. unsetup()?  I think I like the idea of "unwinding" during unsetup(),
> even though it splits the logic a bit.

Never mind, I figured out a way to clean all this up and land the PT interrupt
handler in vmx.c where it belongs.  Getting there is a bit of a journey, but it's
very doable.  That means unwinding in unsetup() is the preferred approach,
otherwise there would be potential for leaving a dangling pointer if a different
vendor module was succesfully loaded.