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[Xen-devel] Re: [patch 2/8] Implement always-locked bit ops, for memory

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [patch 2/8] Implement always-locked bit ops, for memory shared with an SMP hypervisor.
From: Christoph Lameter <clameter@xxxxxxx>
Date: Wed, 2 Aug 2006 18:06:45 -0700 (PDT)
Cc: akpm@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
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On Wed, 2 Aug 2006, Jeremy Fitzhardinge wrote:

> > Would it not be simpler to always use the locked implementation on UP? At
> > least when the kernel is compiled with hypervisor support? This is going to
> > add yet another series of bit operations
> You mean make the standard bit-ops atomic on UP when compiling for
> CONFIG_PARAVIRT?  We think its too much of a burden; there are only a few
> operations which must be locked in the hypervisor case, and bit ops are used
> everywhere.  I'd like to get it to the point where there's no significant
> overhead in configuring for PARAVIRT, even if you run on native hardware.

Thats a good goal but what about the rest of us who have to maintain 
additional forms of bit operations for all architectures. How much is this 
burden? Are locked atomic bitops really that more expensive?

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