linux-kernel - Re: [patch] paravirt: VDSO page is essential

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Message-ID: <45EC1472.3070208@qumranet.com>
Date:	Mon, 05 Mar 2007 15:00:34 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Andi Kleen <ak@...e.de>, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [patch] paravirt: VDSO page is essential

Ingo Molnar wrote:
> * Avi Kivity <avi@...ranet.com> wrote:
>
>   
>>> -#ifdef CONFIG_PARAVIRT
>>> -unsigned int __read_mostly vdso_enabled = 0;
>>> -#else
>>> unsigned int __read_mostly vdso_enabled = 1;
>>> -#endif
>>>       
>
>   
>> Can't paravirt patch the syscall instruction like it does the rest of 
>> the kernel?
>>     
>
> we want to keep the guest as simple and unmodified as possible. And all 
> this #ifdef jungle /will/ bite back. Especially if the change goes in 
> with zero explanation like it did:
>
>     [PATCH] paravirt: Disable vdso by default when CONFIG_PARAVIRT is enabled
>
>     They don't work together and this way even glibc still works.
>
> i rather want an experimental feature (CONFIG_PARAVIRT) broken on some 
> hypervisors for a bit than an entire body of guest OSs getting used to 
> the "you dont have to deal with this VDSO annoyance by default" quirk 
> forever ...
>   

Sure, I agree with this patch.  I'm talking about an alternate solution 
so Xen can work with the vdso instead of #ifdefing away the kernel.

> but yes, i agree that the hypervisor should have the ability to patch 
> the syscall instruction of both the hypervisor interface and of the VDSO 
> interface. But this wasnt implemented like that, and the #ifdef quirk 
> just /prevents/ a sane solution like that from ever getting done the 
> right way.
>   

Rusty, shouldn't this be a one-liner?  No need to involve the hypervisor 
here; the guest can s/syscall/int 80/ on its vdso page like it patches 
cli and its ilk.

-- 
error compiling committee.c: too many arguments to function

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