Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release perf_event per vPMC

[Like Xu]

On 2019/10/8 20:11, Peter Zijlstra wrote:
> 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.

It's reasonable. Thanks.

> 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.

Yes, my union here is wrong and let me fix it in the next version.

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

Yes, we could benefit from 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.