linux-kernel - Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb to compile for 32 bit

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Date:	Wed, 26 Nov 2008 11:53:55 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	Ian.Campbell@...rix.com
Cc:	fujita.tomonori@....ntt.co.jp, jeremy@...p.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	x86@...nel.org
Subject: Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb
	to	compile for 32 bit

On Mon, 24 Nov 2008 11:41:37 +0000
Ian Campbell <Ian.Campbell@...rix.com> wrote:

> On Sat, 2008-11-22 at 10:49 +0900, FUJITA Tomonori wrote:
> > On Fri, 21 Nov 2008 14:21:32 +0000
> > Ian Campbell <Ian.Campbell@...rix.com> wrote:
> > 
> > > 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@...ell.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.
> > 
> > Ah, I vaguely remember this patch. The ~0ULL mask didn't happen here
> > (nobody uses it) so the possibility was false. IMHO, if we use this
> > code on 32bit architectures, the mask should be u64 and the overflow
> > should be handled explicitly. But as you pointed out, looks like that
> > this patch takes account of the overflow.
> 
> Something like this?
> 
> Ian.
> --- 
> 
> swiotlb: explicitly handle segment boundary mask overflow.
> 
> When swiotlb is used on 32 bit we can overflow mask + 1 in the common
> case where mask is 0xffffffffUL. This overflow was previously caught
> by the case which attempts to handle a mask of ~0UL on 64 bit.
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> 
> diff -r 5fa30e5284dd lib/swiotlb.c
> --- a/lib/swiotlb.c	Mon Nov 24 09:39:50 2008 +0000
> +++ b/lib/swiotlb.c	Mon Nov 24 11:37:39 2008 +0000
> @@ -303,7 +303,7 @@
>  	unsigned int nslots, stride, index, wrap;
>  	int i;
>  	unsigned long start_dma_addr;
> -	unsigned long mask;
> +	u64 mask;
>  	unsigned long offset_slots;
>  	unsigned long max_slots;
>  
> @@ -314,6 +314,7 @@
>  	max_slots = mask + 1
>  		    ? ALIGN(mask + 1, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT
>  		    : 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);
> +	BUG_ON(max_slots > 1UL << (BITS_PER_LONG - IO_TLB_SHIFT));

How can this BUG_ON happen? Using u64 for the mask is fine though.

Thanks,
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