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

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Message-ID: <YSZa7dkw6t7yN6vL@google.com>
Date:   Wed, 25 Aug 2021 14:59:57 +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, 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.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b5ade47dad9c..ffc6c2d73508 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11093,6 +11093,7 @@ int kvm_arch_hardware_setup(void *opaque)
 void kvm_arch_hardware_unsetup(void)
 {
        perf_unregister_guest_info_callbacks(&kvm_guest_cbs);
+       kvm_guest_cbs.handle_intel_pt_intr = NULL;

        static_call(kvm_x86_hardware_unsetup)();
 }


vs.


diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b5ade47dad9c..462aa7a360e9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11063,6 +11063,8 @@ int kvm_arch_hardware_setup(void *opaque)

        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;
+       else
+               kvm_guest_cbs.handle_intel_pt_intr = NULL;
        perf_register_guest_info_callbacks(&kvm_guest_cbs);

        if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))

> But I have to say that this fix is much better than the one I proposed [1].
> 
> [1] https://lore.kernel.org/lkml/20210514084436.848396-1-like.xu@linux.intel.com/
> 
> >   	perf_register_guest_info_callbacks(&kvm_guest_cbs);
> >   	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
> >