linux-kernel - Re: [PATCH 5 of 7] x86/paravirt: add register-saving thunks to reduce caller register pressure

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Message-ID: <87fxisvxb9.fsf@basil.nowhere.org>
Date:	Fri, 06 Feb 2009 08:28:26 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Zachary Amsden <zach@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ian Campbell <ian.campbell@...rix.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel@...r.kernel.org,
	Ravikiran Thirumalai <kiran@...lemp.com>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH 5 of 7] x86/paravirt: add register-saving thunks to reduce caller register pressure

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> One of the problems with inserting a pile of C calls where previously
> there were none is that the register pressure is greatly increased.
> The C calling convention says that the caller must expect a certain
> set of registers may be trashed by the callee, and that the callee can
> use those registers without restriction.  This includes the function
> argument registers, and several others.
>
> This patch seeks to alleviate this pressure by introducing wrapper
> thunks that will do the register saving/restoring, so that the
> callsite doesn't need to worry about it, but the callee function can
> be conventional compiler-generated code. 

I had something like this (but not as generic) for the old pre
unification semaphores on x86-64 too. I think it's a generally useful
facility for lots of things (in fact I've been asking gcc developers
for providing this in a general way for some time).

So I would suggest to move the necessary macros to do that outside
paravirt.h into a separate include so that it can be used independently
and not call it PV_*.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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