Subject: Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb
	to	compile for 32 bit
From: Ian Campbell <Ian.Campbell@citrix.com>
To: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: jeremy@goop.org, mingo@elte.hu, linux-kernel@vger.kernel.org,
       xen-devel@lists.xensource.com, x86@kernel.org
In-Reply-To: <20081119112015I.fujita.tomonori@lab.ntt.co.jp>
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	 <20081117124857Z.fujita.tomonori@lab.ntt.co.jp>
	 <1226938566.18916.80.camel@zakaz.uk.xensource.com>
	 <20081119112015I.fujita.tomonori@lab.ntt.co.jp>
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Organization: Citrix Systems, Inc.
Date: Fri, 21 Nov 2008 14:21:32 +0000
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On Wed, 2008-11-19 at 11:19 +0900, FUJITA Tomonori wrote:
> 
> The problem that I talked about in the previous mail:
> 
> >       max_slots = mask + 1
> >                   ? ALIGN(mask + 1, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT
> >                   : 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);
> 
> Since the popular value of the mask is 0xffffffff. So the above code
> (mask + 1 ?) works wrongly if the size of mask is 32bit (well,
> accidentally the result of max_slots is identical though).

I've just been looking at this again and I don't think it is an accident
that this evaluates to the correct value when mask + 1 == 0.

The patch which adds the "mask + 1 ? ... : 1UL << ..." stuff is:

        commit b15a3891c916f32a29832886a053a48be2741d4d
        Author: Jan Beulich <jbeulich@novell.com>
        Date:   Thu Mar 13 09:13:30 2008 +0000
        
            avoid endless loops in lib/swiotlb.c
            
            Commit 681cc5cd3efbeafca6386114070e0bfb5012e249 ("iommu sg merging:
            swiotlb: respect the segment boundary limits") introduced two
            possibilities for entering an endless loop in lib/swiotlb.c:
            
             - if max_slots is zero (possible if mask is ~0UL)
             [...]

I think the existing code is the nicest way to handle this corner case
and it is necessary anyway to handle the ~0UL case on 64 bit.

Ian.

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