linux-kernel - Re: [PATCH v2 6/8] iommu: Move IOMMU pagesize check to attach

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Message-ID: <ede06b33-38ff-1ba2-c32c-c5bb3aaddb62@arm.com>
Date:   Wed, 1 Sep 2021 19:53:23 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>
Cc:     iommu@...ts.linux-foundation.org, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...nel.org>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Alexander Graf <graf@...zon.com>,
        Hector Martin <marcan@...can.st>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/8] iommu: Move IOMMU pagesize check to attach_device

On 2021-09-01 18:14, Sven Peter wrote:
> 
> 
> On Tue, Aug 31, 2021, at 23:39, Alyssa Rosenzweig wrote:
>>> +	if ((1 << __ffs(domain->pgsize_bitmap)) > PAGE_SIZE) {
>>
>> Not a fan of this construction. Could you assign `(1 <<
>> __ffs(domain->pgsize_bitmap))` to an appropriately named temporary (e.g
>> min_io_pgsize) so it's clearer what's going on?
> 
> Good point, will do that for the next version.

Or maybe just test "__ffs(domain->pgsize_bitmap) > PAGE_SHIFT", or 
perhaps even avoid shifts altogether with something like 
"domain->pgsize_bitmap & (PAGE_SIZE | PAGE_SIZE - 1)".

Robin.

> 
>>
>>> +		pr_warn("IOMMU page size cannot represent CPU pages.\n");
>>
>> "Represent" how?
>>
> 
> Looks like I dropped an "exactly" there when taking this line from iova.c :)
> 
> 
> 
> Thanks,
> 
> 
> Sven
>