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

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Date:   Tue, 8 Oct 2019 14:11:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.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 Tue, Oct 01, 2019 at 08:33:45PM +0800, Like Xu wrote:
> Hi Peter,
> 
> On 2019/10/1 16:23, Peter Zijlstra wrote:
> > On Mon, Sep 30, 2019 at 03:22:57PM +0800, Like Xu wrote:
> > > +	union {
> > > +		u8 event_count :7; /* the total number of created perf_events */
> > > +		bool enable_cleanup :1;
> > 
> > That's atrocious, don't ever create a bitfield with base _Bool.
> 
> I saw this kind of usages in the tree such as "struct
> arm_smmu_master/tipc_mon_state/regmap_irq_chip".

Because other people do tasteless things doesn't make it right.

> I'm not sure is this your personal preference or is there a technical
> reason such as this usage is not incompatible with union syntax?

Apparently it 'works', so there is no hard technical reason, but
consider that _Bool is specified as an integer type large enough to
store the values 0 and 1, then consider it as a base type for a
bitfield. That's just disguisting.

Now, I suppose it 'works', but there is no actual benefit over just
using a single bit of any other base type.

> My design point is to save a little bit space without introducing
> two variables such as "int event_count & bool enable_cleanup".

Your design is questionable, the structure is _huge_, and your union has
event_count:0 and enable_cleanup:0 as the same bit, which I don't think
was intentional.

Did you perhaps want to write:

	struct {
		u8 event_count : 7;
		u8 event_cleanup : 1;
	};

which has a total size of 1 byte and uses the low 7 bits as count and the
msb as cleanup.

Also, the structure has plenty holes to stick proper variables in
without further growing it.

> By the way, is the lazy release mechanism looks reasonable to you?

I've no idea how it works.. I don't know much about virt.