linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Thu, 10 Jul 2008 13:54:47 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>   
>> No, that sounds like a bad idea.  For one, how would you enforce it?  How would
>> you check for it?  It's one of those things that would mostly work and then fail
>> very rarely.
>>     
>
> Well the easiest way would be to avoid the letting people take the address of
> per cpu memory, and just provide macros to read/write it.  We are 90% of the
> way there already so it isn't a big jump.
>   

Well, the x86_X_percpu api is there.  But per_cpu() and get_cpu_var() 
both explicitly return lvalues which can have their addresses taken.

>> Secondly, I depend on it.  I register a percpu structure with Xen to share
>> per-vcpu specific information (interrupt mask, time info, runstate stats, etc).
>>     
>
> Well even virtual allocation is likely to break the Xen sharing case as you
> would at least need to compute the physical address and pass it to Xen.
>   

Right.  At the moment it assumes that the percpu variable is in the 
linear mapping, but it could easily do a pagetable walk if necessary.

    J
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