linux-kernel - Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support

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Message-ID: <47C8717F.3030009@zytor.com>
Date:	Fri, 29 Feb 2008 12:56:31 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>, Zachary Amsden <zach@...are.com>
Subject: Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support
 - alternatives"

Jeremy Fitzhardinge wrote:
> The patch "x86 - Enhance DEBUG_RODATA support - alternatives" enables 
> the kernel for writing by clearing X86_CR0_WP allow privileged writes.  
> This won't work in a paravirt environment for two reasons:
> 
>   1. the kernel may not be running in ring 0, so writes will still be
>      prevented
>   2. the hypervisor prevents X86_CR0_WP from being cleared anyway (it
>      GPFs the cr0 update)
> 
> This crashes on Xen, and it would probably break VMI too.
> 
> The only safe way to allow writes is to change the page permissions 
> (either on the page itself, or create a temporary writable alias for 
> that page).  Perhaps something you could do it with kmap_atomic.
> 

A properly implemented hypervisor should arguably emulate this.

Doesn't really mean the patch is worth the pain.

	-hpa
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