Re: [PATCH 01/50] x86/boot/e820: Introduce arch/x86/include/asm/e820/types.h

From: Ingo Molnar <mingo@kernel.org>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Yinghai Lu <yinghai@kernel.org>,
	Arnaldo Carvalho de Melo <acme@infradead.org>
Subject: Re: [PATCH 01/50] x86/boot/e820: Introduce arch/x86/include/asm/e820/types.h
Date: Tue, 31 Jan 2017 17:35:08 +0100	[thread overview]
Message-ID: <20170131163508.GA16162@gmail.com> (raw)
In-Reply-To: <20170131054124.GA21189@ravnborg.org>

* Sam Ravnborg <sam@ravnborg.org> wrote:

> Obvious when there are two vastly different solutions there are some
> middle ground solutions too.
>
> And perf implements a tool based method to check that the manually 
> copied/created/maintained headers are in sync with the kernel. which is just 
> another way to maintain a set of copied headers.
> 
> But this does not at all address the point which is that it is an unessesary 
> burden to put on all the _users_ of the kernel uapi headers to amintain their 
> own copy of the kernel headers (tool assisted or not).

In 99% of the cases it's trivial to keep them updated on the tooling side. 
Basically a copy only has to be made if a change is done - and changes are not 
that common in the ABI space.

> The rationale behind requesting the users of the kernel uapi headers seems to be 
> that the kernel people would like to have the freedom to fix wrong doings in the 
> past without violatign the uapi.
>
> So the benefit here is simpler headers on the kernel side compared
> to all users of kernel uapi heders to manually (tool assisted or not)
> maintaining their own copies of the uapi kernel headers.

I was reacting to this original claim of yours:

> > > The idea with uapi is the the kernel provides a sane set of headers to be 
> > > used by user space.
> > >
> > > So we avoid random copies that is maintained by random people in random ways 
> > > resulting in random bugs.

There's no 'random copies' - they are copies of past, valid versions of the kernel 
headers, and there's a mechanism to update them to the latest.

> Heck - if this is how it should be done then the the kernel
> should provide the copy of the headers that describe the
> uapi and the kernel should have the tool infrastructure to check
> that the headers are OK.
> Not all the users outside the kernelspace.
> 
> So just to repeat - it is an error prone design to let users
> of the kernel uapi maintain their own copies of the kernel
> uapi header. It is the job of the kernel.

But "random copies" is not what perf does. Tell me, how is the perf mechanism of 
using the headers "error-prone"? It's a delayed COW mechanism - COW is not an 
error-prone concept in any way ...

Thanks,

	Ingo