linux-kernel - Re: [PATCH] iommu-common: Do not use 64 bit constant 0xffffffffffffffffl for computing align

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Message-ID: <874mkzevz4.fsf@rasmusvillemoes.dk>
Date:	Sun, 19 Jul 2015 22:25:03 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	benh@...nel.crashing.org, davem@...emloft.net
Subject: Re: [PATCH] iommu-common: Do not use 64 bit  constant 0xffffffffffffffffl for computing align_mask

On Sun, Jul 19 2015, Guenter Roeck <linux@...ck-us.net> wrote:

> On Sun, Jul 19, 2015 at 02:20:14PM +0200, Sowmini Varadhan wrote:
>> 
>> Using a 64 bit constant generates "warning: integer constant is too
>> large for 'long' type" on 32 bit platforms. Instead use ~0l to get
>> the desired effect.
>> 
>> Detected by Andrew Morton who has confirmed that this patch
>> fixes the warning on i386/gcc-4.4.3, i386/gcc-4.4.0 and arm/gcc-4.4.4.
>> 
>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>> ---
>>  lib/iommu-common.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>> 
>> diff --git a/lib/iommu-common.c b/lib/iommu-common.c
>> index df30632..fd1297d 100644
>> --- a/lib/iommu-common.c
>> +++ b/lib/iommu-common.c
>> @@ -119,7 +119,7 @@ unsigned long iommu_tbl_range_alloc(struct device *dev,
>>  	unsigned long align_mask = 0;
>>  
>>  	if (align_order > 0)
>> -		align_mask = 0xffffffffffffffffl >> (64 - align_order);
>> +		align_mask = ~0l >> (64 - align_order);
>>  
> Wonder if this just hides the real problem. Unless align_order
> is very large, the resulting mask on 32 bit systems may be 0.
> Is this really the idea ?

Probably not, but that's not what would happen on x86: the shift
only depends on the lower 5 or 6 bits - I don't know if other platforms
also has that behaviour. So for align_order == 2 and x86_32 we'd
effectively end up with a shift of 62 & 31 == 30 (though technically
undefined behaviour), and the desired mask of 0x3.

Wouldn't GENMASK(align_order-1, 0) work for all cases (assuming
align_order has a sane value)?

Rasmus
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