linux-kernel - Re: [PATCH v1 1/1] IB/hfi1: Don't cast parameter in bit operations

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Message-ID: <01f0c30b-08dd-e70c-ddfa-b47843e76366@cornelisnetworks.com>
Date:   Thu, 24 Feb 2022 08:44:29 -0500
From:   Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>
To:     'Andy Shevchenko' <andriy.shevchenko@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Marciniszyn <mike.marciniszyn@...nelisnetworks.com>
Subject: Re: [PATCH v1 1/1] IB/hfi1: Don't cast parameter in bit operations

On 2/23/22 5:30 PM, 'Andy Shevchenko' wrote:
> On Wed, Feb 23, 2022 at 09:44:32PM +0000, David Laight wrote:
>> From: Andy Shevchenko
>>> Sent: 23 February 2022 18:54
>>>
>>> While in this particular case it would not be a (critical) issue,
>>> the pattern itself is bad and error prone in case somebody blindly
>>> copies to their code.
>>
>> It is horribly wrong on BE systems.
> 
> You mean the pattern? Yes, it has three issues regarding to endianess and
> potential out of boundary access.
> 
> ...
> 
>>> -	return handled;
>>> +	return IRQ_RETVAL(!bitmap_empty(pending, CCE_NUM_INT_CSRS * 64));
> 
>> You really don't want to scan the bitmap again.
> 
> Either way it wastes cycles, the outcome depends on the actual distribution of
> the interrupts across the bitmap. If it gathered closer to the beginning of the
> bitmap, my code wins, otherwise the original ones.
> 
>> Actually, of the face of it, you could merge the two loops.
>> Provided you clear the status bit before calling the relevant
>> handler I expect it will all work.
> 
> True. I will consider that for v2.

Will wait for a v2 patch and I'll test it. We are very sensitive to performance
changes.

-Denny