linux-kernel - Re: [patch V4 part 2 13/18] x86/kvm: Move context tracking where it belongs

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Message-ID: <87pnbd4du2.fsf@nanos.tec.linutronix.de>
Date:   Sat, 09 May 2020 12:12:05 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 2 13/18] x86/kvm: Move context tracking where it belongs

Andy Lutomirski <luto@...nel.org> writes:
> On Tue, May 5, 2020 at 7:14 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> Context tracking for KVM happens way too early in the vcpu_run()
>> code. Anything after guest_enter_irqoff() and before guest_exit_irqoff()
>> cannot use RCU and should also be not instrumented.
>>
>> The current way of doing this covers way too much code. Move it closer to
>> the actual vmenter/exit code.
>
> Now you've made me wonder what happens if someone traces
> vmx_vcpu_run().  I'm not sure I really want to think about this.

Been there, done that. Kinda worked but adding a kprobe into the guts of
it made it go sideways very fast.