linux-kernel - Re: [PATCH v2 5/6] kvm: x86: Emulate MSR IA32_CORE

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Date:   Mon, 10 Feb 2020 19:52:13 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v2 5/6] kvm: x86: Emulate MSR IA32_CORE_CAPABILITIES

On Tue, Feb 4, 2020 at 1:37 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Feb 04, 2020 at 05:19:26PM +0800, Xiaoyao Li wrote:
>
> > > > + case MSR_IA32_CORE_CAPS:
> > > > +         if (!msr_info->host_initiated)
> > >
> > > Shouldn't @data be checked against kvm_get_core_capabilities()?
> >
> > Maybe it's for the case that userspace might have the ability to emulate SLD
> > feature? And we usually let userspace set whatever it wants, e.g.,
> > ARCH_CAPABILITIES.
>
> If the 'sq_misc.split_lock' event is sufficiently accurate, I suppose
> the host could use that to emulate the feature at the cost of one
> counter used.

I would be impressed if the event were to fire before executing the
offending split lock.  Wouldn't the best possible result be for it to
fire with RIP pointing to the *next* instruction?  This seems like it
could be quite confusing to a guest.