linux-kernel - Re: [PATCH] srcu: Isolate srcu sections using CONFIG

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Date:	Mon, 29 Dec 2014 23:32:10 -0500
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"supporter:S390" <linux390@...ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Anton Blanchard <anton@...ba.org>,
	Scott Wood <scottwood@...escale.com>,
	Andy Fleming <afleming@...escale.com>,
	Tiejun Chen <tiejun.chen@...driver.com>,
	Daniel Walter <dwalter@...gle.com>,
	Jens Freimann <jfrei@...ux.vnet.ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"open list:LINUX FOR POWERPC..." <linuxppc-dev@...ts.ozlabs.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:S390" <linux-s390@...r.kernel.org>
Subject: Re: [PATCH] srcu: Isolate srcu sections using CONFIG_SRCU

On Mon, Dec 29, 2014 at 5:03 AM, Martin Schwidefsky
<schwidefsky@...ibm.com> wrote:
> On Sat, 27 Dec 2014 12:17:43 -0500
> Pranith Kumar <bobby.prani@...il.com> wrote:
>
>> @@ -65,10 +65,13 @@
>>  #include <asm/kexec.h>
>>  #include <asm/mmu_context.h>
>>  #include <asm/code-patching.h>
>> -#include <asm/kvm_ppc.h>
>>  #include <asm/hugetlb.h>
>>  #include <asm/epapr_hcalls.h>
>>
>> +#if IS_ENABLED(CONFIG_KVM)
>> +#include <asm/kvm_ppc.h>
>> +#endif
>> +
>>  #ifdef DEBUG
>>  #define DBG(fmt...) udbg_printf(fmt)
>>  #else
>
> I always cringe when I see an include protected by an #ifdef.
> Is this really necessary? All that is done in asm-offsets.c is
> to calculate offsets, the code where the two offsets in question
> are used (entry64.S) does have the #ifdef for CONFIG_KVM.

I agree that this is not the ideal way to do this. But, it has been
the way things were already being done. If you see
arch/powerpc/kernel/asm-offsets.c, there are quite some includes which
are within ifdefs.

I've considered other alternatives (though not in-depth) and found
that they will require quite some refactoring. One simple idea is to
move this #ifdef to within kvm_ppc.h. That should make the inclusion
of this file a no-op in all the places where this is being included
without KVM being enabled. But I am not 100% sure of that approach.

Any suggestions are welcome.

-- 
Pranith
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