linux-kernel - Re: [patch 12/17] Consistently wrap paravirt ops callsites to make them patchable

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Date:	Mon, 2 Apr 2007 09:11:50 +0200
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	virtualization@...ts.osdl.org, lkml <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zachary Amsden <zach@...are.com>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [patch 12/17] Consistently wrap paravirt ops callsites to make them patchable

On Monday 02 April 2007 07:57, Jeremy Fitzhardinge wrote:
> Wrap a set of interesting paravirt_ops calls in a wrapper which makes
> the callsites available for patching.  Unfortunately this is pretty
> ugly because there's no way to get gcc to generate a function call,
> but also wrap just the callsite itself with the necessary labels.
> 
> This patch supports functions with 0-4 arguments, and either void or
> returning a value.  64-bit arguments must be split into a pair of
> 32-bit arguments (lower word first).  Small structures are returned in
> registers.

Can you please add some comments to the code explaining this a little?
Best would be perhaps a overview document in Documentation too.

> +#define PVOP_CALL0(__rettype, __op)					\

The __s shouldn't be needed for the macro arguments because
there is no shared name space with the caller.

> +	({								\
> +		__rettype __ret;					\
> +		if (sizeof(__rettype) > sizeof(unsigned long)) {	\
> +			unsigned long long __tmp;			\
> +			unsigned long __ecx;				\
> +			asm volatile(paravirt_alt(PARAVIRT_CALL)	\

Not having the volatile would probably generate better code, but it 
seems much safer for now.

> +				     : "=A" (__tmp), "=c" (__ecx)	\
> +				     : paravirt_type(__op),		\
> +				       paravirt_clobber(CLBR_ANY)	\
> +				     : "memory", "cc");			\

And the cc clobber is also not needed

-Andi
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