linux-kernel - Re: [GIT PULL] x86/paravirt for v2.6.33

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Message-ID: <alpine.LFD.2.00.0912091354190.3560@localhost.localdomain>
Date:	Wed, 9 Dec 2009 13:58:33 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickens <hugh@...itas.com>
Subject: Re: [GIT PULL] x86/paravirt for v2.6.33

On Wed, 9 Dec 2009, Jeremy Fitzhardinge wrote:

> On 12/08/09 13:34, Linus Torvalds wrote:
> > I do _not_ want to add any more task_pt_regs() crap, please.
> > 
> > Why? It's wrong for at least vm86 mode (and from kernel system calls).
> >    
> 
> Would the stack frame version work in these cases?

It would "work" in the sense that at least it wouldn't corrupt the "outer" 
stack frame - it would only change the inner one. For vm86 mode, that 
would actually matter (iopl is meaningful), but as Peter also said, I 
don't think we actually allow direct system calls from vm86 mode.

For me it's actually more of a conceptual complaint: I really think 
'task_pt_regs()' is only reliable for ptrace and is simply _wrong_ in 
other situations. On other architectures, you literally need to set up the 
stack _differently_ on the signal handling path - which is what ptrace 
does - than on regular system call paths.

So conceptually, the system call stack layout is simply _different_ than 
the ptrace stack. And I'd hate to have x86 code that teaches people to do 
things that really don't work in general.

			Linus
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