linux-kernel - Re: [RFC PATCH 15/33] move segment checks to subarch

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Date:	Tue, 18 Jul 2006 04:28:01 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Chris Wright <chrisw@...s-sol.org>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [RFC PATCH 15/33] move segment checks to subarch

Arjan van de Ven wrote:
> On Tue, 2006-07-18 at 00:00 -0700, Chris Wright wrote:
>   
>> plain text document attachment (i386-segments)
>> We allow for the fact that the guest kernel may not run in ring 0.
>> This requires some abstraction in a few places when setting %cs or
>> checking privilege level (user vs kernel).
>>     
>
>   
>> -	regs.xcs = __KERNEL_CS;
>> +	regs.xcs = get_kernel_cs();
>>     
>
> Hi,
>
> wouldn't this patch be simpler if __KERNEL_CS just became the macro that
> currently is get_kernel_cs() for the XEN case? then code like this
> doesn't need changing at all...
>   

The tradeoff is that then you can't use __KERNEL_CS is assembler code, 
and it is used in entry.S to detect NMI / debug trap workarounds - which 
don't actually need to be paravirtualized, as it is easier to hide the 
nasty cases which cause those side effects.

Zach
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