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

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Message-ID: <20191009081602.GI2328@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Oct 2019 10:16:02 +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 09:15:03AM +0200, Paolo Bonzini wrote:
> For stuff like hardware registers, bitfields are probably a bad idea
> anyway, so let's only consider the case of space optimization.

Except for hardware registers? I actually like bitfields to describe
hardware registers.

> bool:2 would definitely cause an eyebrow raise, but I don't see why
> bool:1 bitfields are a problem.  An integer type large enough to store
> the values 0 and 1 can be of any size bigger than one bit.

Consider:

	bool	foo:1;
	bool	bar:1;

Will bar use the second bit of _Bool? Does it have one? (yes it does,
but it's still weird).

But worse, as used in the parent thread:

	u8	count:7;
	bool	flag:1;

Who says the @flag thing will even be the msb of the initial u8 and not
a whole new variable due to change in base type?

> 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.

The _only_ thing about _Bool is that it magically casts values to 0,1.
Single bit bitfield variables have no choice but to already be in that
range.

So expressions where it matters are:

	x = (7&2)	// x == 2
vs
	x = !!(7&2)	// x == 1

But it is impossible for int:1 and _Bool to behave differently.

> However, in this patch bitfields are unnecessary and they result in
> worse code from the compiler.

Fully agreed :-)