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From: Jeremy Fitzhardinge <jeremy@goop.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	Ingo Molnar <mingo@elte.hu>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@suse.de>, Zachary Amsden <zach@vmware.com>
Subject: Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support - alternatives"
Date: Fri, 29 Feb 2008 13:09:28 -0800	[thread overview]
Message-ID: <47C87488.9030205@goop.org> (raw)
In-Reply-To: <47C8717F.3030009@zytor.com>

H. Peter Anvin wrote:
> 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.

(lguest too, of course)

>> 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.

No, it would be irritating to implement.

Seems to me that doing the update in a temporary kmap_atomic mapping 
would be a more straightforward way to go, anyway.  How would you 
implement this on a processor without something like X86_CR0_WP?

    J

  reply	other threads:[~2008-02-29 21:13 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-29 20:39 Jeremy Fitzhardinge
2008-02-29 20:56 ` H. Peter Anvin
2008-02-29 21:09   ` Jeremy Fitzhardinge [this message]
2008-03-03 17:30     ` Mathieu Desnoyers
2008-03-03 17:35       ` Andi Kleen
2008-03-03 17:58       ` Jeremy Fitzhardinge
2008-03-03 18:03         ` Andi Kleen
2008-03-03 18:05           ` Jeremy Fitzhardinge
2008-03-03 20:53             ` Mathieu Desnoyers
2008-03-03 22:15               ` Jeremy Fitzhardinge
2008-03-04  4:12                 ` Mathieu Desnoyers
2008-02-29 21:24 ` Ingo Molnar

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