linux-kernel - Re: [PATCH v2 1/4] x86/hyper-v: move synic/stimer control structures definitions to hyperv-tlfs.h

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Message-Id: <8A215F49-BB8F-4E93-AC62-EC33B4734F24@gmail.com>
Date:   Tue, 27 Nov 2018 17:49:29 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Roman Kagan <rkagan@...tuozzo.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 1/4] x86/hyper-v: move synic/stimer control structures
 definitions to hyperv-tlfs.h

> On Nov 27, 2018, at 10:48 AM, Roman Kagan <rkagan@...tuozzo.com> wrote:
> 
> On Tue, Nov 27, 2018 at 02:10:49PM +0100, Vitaly Kuznetsov wrote:
>> Roman Kagan <rkagan@...tuozzo.com> writes:
>>> On Mon, Nov 26, 2018 at 04:47:29PM +0100, Vitaly Kuznetsov wrote:
>>> I personally tend to prefer masks over bitfields, so I'd rather do the
>>> consolidation in the opposite direction: use the definitions in
>>> hyperv-tlfs.h and replace those unions/bitfields elsewhere.  (I vaguely
>>> remember posting such a patchset a couple of years ago but I lacked the
>>> motivation to complete it).
>> 
>> Are there any known advantages of using masks over bitfields or the
>> resulting binary code is the same?
> 
> Strictly speaking bitwise ops are portable while bitfields are not, but
> I guess this is not much of an issue with gcc which is dependable to
> produce the right thing.
> 
> I came to dislike the bitfields for the false feeling of atomicity of
> assignment while most of the time they are read-modify-write operations.
> 
> And no, I don't feel strong about it, so if nobody backs me on this I
> give up :)

Last time I tried to push a change from bitmasks to bitfields I failed:
https://lkml.org/lkml/2014/9/16/245

On a different note: how come all of the hyper-v structs are not marked
with the “packed" attribute?