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

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Message-ID: <3f9c6787-6fe9-0867-3e85-d3fb661484d4@linux.intel.com>
Date:   Wed, 9 Oct 2019 16:07:13 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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

Hi Paolo,
On 2019/10/9 15:15, Paolo Bonzini wrote:
> On 09/10/19 05:14, Like Xu wrote:
>>>
>>>
>>>> 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.
> 
> /me chimes in since this is KVM code after all...
> 
> For stuff like hardware registers, bitfields are probably a bad idea
> anyway, so let's only consider the case of space optimization.
> 
> 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.
> 
> 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);
> 
> which can be a plus or a minus depending on the point of view. :)
> Either way, bool bitfields are useful if you are using bitfields for
> space optimization, especially if you have existing code using bool and
> it might rely on the idiom above.
> 
> However, in this patch bitfields are unnecessary and they result in
> worse code from the compiler.  There is plenty of padding in struct
> kvm_pmu, with or without bitfields, so I'd go with "u8 event_count; bool
> enable_cleanup;" (or better "need_cleanup").

Thanks. The "u8 event_count; bool need_cleanup;" looks good to me.

So is the lazy release mechanism looks reasonable to you ?
If so, I may release the next version based on current feedback.

> 
> Thanks,
> 
> Paolo
>