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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> >         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@xxxxxxxxxx>

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));
 
        /*
         * For mappings greater than a page, we limit the stride (and




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