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

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Date:	Thu, 10 Jul 2008 13:22:52 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

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.

> 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.

Eric
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