linux-kernel - Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release perf

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Message-ID: <20191009093210.GK2328@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Oct 2019 11:32:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Like Xu <like.xu@...ux.intel.com>, kvm@...r.kernel.org,
        rkrcmar@...hat.com, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        ak@...ux.intel.com, wei.w.wang@...el.com, kan.liang@...el.com,
        like.xu@...el.com, ehankland@...gle.com, arbel.moshe@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release
 perf_event per vPMC

On Wed, Oct 09, 2019 at 11:21:30AM +0200, Paolo Bonzini wrote:
> On 09/10/19 10:16, Peter Zijlstra wrote:

> >> bool bitfields preserve the magic behavior where something like this:
> >>
> >>   foo->x = y;
> >>
> >> (x is a bool bitfield) would be compiled as
> >>
> >>   foo->x = (y != 0);
> > 
> > This is confusion; if y is a single bit bitfield, then there is
> > absolutely _NO_ difference between these two expressions.
> 
> y is not in a struct so it cannot be a single bit bitfield. :) If y is
> an int and foo->x is a bool bitfield, you get the following:
> 
> 	foo->x = 6;	/* foo->x is 1, it would be 0 for int:1 */
> 	foo->x = 7;	/* foo->x is 1, it would be 1 for int:1 */
> 

Urgh, reading hard. You're right!