linux-kernel - Re: [PATCH 1/19] unify desc

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Message-ID: <475871A9.5050707@goop.org>
Date:	Thu, 06 Dec 2007 14:03:21 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Glauber de Oliveira Costa <glommer@...il.com>
CC:	Andi Kleen <ak@...e.de>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu, ehabkost@...hat.com,
	avi@...ranet.com, anthony@...emonkey.ws,
	virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	chrisw@...s-sol.org, rostedt@...dmis.org, hpa@...or.com,
	zach@...are.com
Subject: Re: [PATCH 1/19] unify desc_struct

Glauber de Oliveira Costa wrote:
> On Dec 6, 2007 6:54 PM, Andi Kleen <ak@...e.de> wrote:
>   
>>> +/*
>>> + * FIXME: Acessing the desc_struct through its fields is more elegant,
>>> + * and should be the one valid thing to do. However, a lot of open code
>>> + * still touches the a and b acessors, and doing this allow us to do it
>>> + * incrementally. We keep the signature as a struct, rather than an union,
>>> + * so we can get rid of it transparently in the future -- glommer
>>> + */
>>> +#define raw_desc_struct struct { unsigned int a, b; }
>>> +#define detailed_desc_struct                                   \
>>> +  struct {                                                       \
>>> +     u16 limit0;                                             \
>>> +     u16 base0;                                              \
>>> +     unsigned base1 : 8, type : 4, s : 1, dpl : 2, p : 1;    \
>>> +     unsigned limit : 4, avl : 1, l : 1, d : 1, g : 1, base2 :8;\
>>> +  }
>>>       
>> The standard clean way to do this is with a anonymous union.
>>     
> It is an anonymous union.
>
> However:
>
> * It's an union of structs
> * I wished to keep the toplevel type as a struct
> The alternative would be to write:
>
> struct desc_struct {
> union {
>     struct { unsigned int a, b; };
>     struct {
>                u16 limit0;
>                u16 base0;
>                unsigned base1 : 8, type : 4, s : 1, dpl : 2, p : 1;
>                unsigned limit : 4, avl : 1, l : 1, d : 1, g : 1, base2 :8;
>     };
> };
> };
>
> Which is fine, it's all the same in the end. Just with more shift
> rights, and more visual pollution.
>   

No, that's much clearer.  It's a pity that the anonymous struct/union
syntax isn't general enough to allow:

struct desc_packed {
               u16 limit0;
               u16 base0;
               unsigned base1 : 8, type : 4, s : 1, dpl : 2, p : 1;
               unsigned limit : 4, avl : 1, l : 1, d : 1, g : 1, base2 :8;
};

struct desc {
	struct desc_packed;
};

    J
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