linux-kernel - Re: [PATCH 4/7] x86/vm86: Move vm86 fields out of thread

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Message-ID: <20150823115113.GA26296@gmail.com>
Date:	Sun, 23 Aug 2015 13:51:13 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Brian Gerst <brgerst@...il.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 4/7] x86/vm86: Move vm86 fields out of thread_struct


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, Aug 5, 2015 at 10:48 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >>
> >> Some of these field names are visible to userspace and can't change.
> >
> > That's a misconception: bits in the uapi headers can be renamed just fine.
> 
> I disagree. If it causes pain for user space, we just shouldn't do it.

Ok, agreed - I wanted to argue against the "can't change" statement and went 
overboard with my own statement ...

Quite often headers can change and we've changed a number of fields in the past - 
but if they cause pain (as in this case) we don't do it.

So I'd say that based on past experience:

 - changing small details in uapi headers is usually fine.
 - changing small details in the ABI is usually not fine.

If anything breaks then the policy is the same, regardless of likelihood: 
reverting the change.

> Some people copy the headers (preferred), others include the kernel header 
> directory with a include path or a symlink, and it's damn painful if we start 
> changing things that user space depends on.
> 
> I'd say that it's like the ABI - if people don't really notice, you can do it, 
> but if it breaks the build of a user app, we should be very very careful. The 
> breakage is often hard to fix because of nasty versioning issues..
> 
> Our goal in life really is "don't screw up user space".

Yeah, agreed 100%.

Thanks,

	Ingo
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