linux-kernel - Re: [PATCH/resend/bypass] um: Preinclude include/linux/kern

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Date:	Tue, 25 Sep 2012 22:37:13 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Joe Perches <joe@...ches.com>,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/resend/bypass] um: Preinclude include/linux/kern_levels.h

On Tue, Sep 25, 2012 at 9:43 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Sep 25, 2012 at 12:20:55PM -0700, Linus Torvalds wrote:
>> IOW, this part of the patch:
>>
>> -       c_flags = -Wp,-MD,$(depfile) $(USER_CFLAGS) -include user.h
>> $(CFLAGS_$(basetarget).o)
>> +       c_flags = -Wp,-MD,$(depfile) $(USER_CFLAGS) -include
>> $(srctree)/include/linux/kern_levels.h -include user.h
>> $(CFLAGS_$(basetarget).o)
>>
>> just makes me go want to puke. The user.h file already has other
>> #include's in it, so I really don't see why you create this insane
>> special case.
>>
>> And why does UM have those "UM_KERN_XYZ" defines in the first place?
>> Why isn't it just using KERN_XYZ directly? Is it because kern_levels.h
>> didn't use to exist, so it was some kind of "let's create our own that
>> we can hide in our special headers".
>
> Because user.h is included *without* kernel headers in include path.

Indeed.

> It's for the stuff that is compiled with host libc headers.  Keep in
> mind that UML talks to libc like normal architecture would talk to
> hardware.  IOW, analogs of asm glue are in (host) userland C.  And
> they need libc headers instead of the kernel ones.  That's what that
> USER_OBJ thing is about.  Kernel-side constants, etc. are delivered
> to that sucker using the same mechanism we normally use to give them
> to assembler - asm-offsets.c.  And here, of course, slapping ifndef
> __ASSEMBLER__ around the tricky bits will not work - the header itself
> is just fine, but getting kernel headers in the search path really
> isn't.
>
>         I agree that proposed solution is ugly.  What we can do is copy
> the damn header into include/generated and #include <generated/kern_levels.h>
> from user.h.  And kill UM_KERN_... stuff.  Objections?

My first submission had "We may convert all UM_KERN_* users to KERN_*
and drop the extra defines?" as a suggestion, but so far I haven't found time
to implement that...

Still, no one came up with a better patch, and this is a regression.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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