Re: [PATCH v2 2/4] x86/boot: Use header to allows inclusion of public xen.h header

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.12.2024 15:35, Frediano Ziglio wrote:
> On Tue, Dec 10, 2024 at 10:32 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 22.11.2024 10:33, Frediano Ziglio wrote:
>>> This allows to include other headers and avoid duplicated declarations.
>>>
>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>>
>> Again it's left unclear what the purpose / goal is.
>>
> 
> Reduce duplication avoiding duplicate declarations. The alternative
> would be to duplicate them, which was proposed already and refused as
> duplication was not good.

Which declarations specifically?

>>> --- /dev/null
>>> +++ b/xen/arch/x86/include/boot/public/xen.h
>>> @@ -0,0 +1,28 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>> +
>>> +/* This header allows the inclusion of public xen.h */
>>> +
>>> +#ifndef BOOT__PUBLIC__XEN_H
>>> +#define BOOT__PUBLIC__XEN_H
>>> +
>>> +#if !defined(__XEN__) || defined(__XEN_TOOLS__) || __XEN__ != 1
>>> +#error Unexpected defines
>>> +#endif
>>
>> What is this to guard against? We're in the Xen tree, building Xen.
>>
> 
> In include/public/arch-x86/xen.h file there are these declarations:
> 
> #if defined(__i386__)
> # ifdef __XEN__
> __DeFiNe__ __DECL_REG_LO8(which) uint32_t e ## which ## x
> __DeFiNe__ __DECL_REG_LO16(name) union { uint32_t e ## name; }
> # endif
> #include "xen-x86_32.h"
> # ifdef __XEN__
> __UnDeF__ __DECL_REG_LO8
> __UnDeF__ __DECL_REG_LO16
> __DeFiNe__ __DECL_REG_LO8(which) e ## which ## x
> __DeFiNe__ __DECL_REG_LO16(name) e ## name
> # endif
> #elif defined(__x86_64__)
> #include "xen-x86_64.h"
> #endif
> 
> This header allows us to include that part without compiler errors due
> to __DeFiNe__ and __UnDeF__ not being C code.

And why exactly can't 32-bit code simply include the compat variant of
the public header, which is being generated by processing those non-C
constructs?

Jan