linux-kernel - Re: [PATCH 1/4] x86 paravirt_ops: create no

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Date:	Sun, 06 Aug 2006 23:23:49 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@....de>
CC:	virtualization@...ts.osdl.org, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH 1/4] x86 paravirt_ops: create no_paravirt.h for native
 ops

Andi Kleen wrote:
>> so it would be  
>> nice to use one of the other serializing instructions in this case.
>>     
>
> You would first need to find one that works in ring 3. On x86-64 it is 
> used in the gettimeoday vsyscall in ring 3 to synchronize the TSC and 
> afaik John was about to implement that for i386 too.
>   

Well, that's really usermode code, so I don't think we'd necessarily 
touch it at all.  It's not the same problem as the (single, at the 
moment) ring 0 use.

> BTW another issue that I haven't checked but we will need to make
> this also an alternative() for another case - it is faily important
> to patch it out on Intel systems with a synchronized TSC where it is
> fairly expensive. That is also not done yet on i386, but will be 
> likely once vsyscall gettimeofday is implemented.
>
> So basically you would need double patching. Ugly.
>   

Yeah.  I guess the cleanest way to do that is do the paravirt 
substitution, and then nop it out later if it isn't needed in the vsyscall.

> I would recommend to keep it out of para ops.

It's hardly a big deal either way.  There's only one in-kernel use of it.

    J
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