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    SubjectRe: [PATCH 2/4] iovmm: fix roundup for next area and end check for the last area
    On Fri, Oct 01, 2010 at 09:21:36PM +0200, ext Guzman Lugo, Fernando wrote:
    >
    > > On Fri, Oct 01, 2010 at 06:10:30PM +0200, ext Guzman Lugo,
    > > Fernando wrote:
    > > >
    > >
    > > [snip]
    > >
    > > > > > arch/arm/plat-omap/iovmm.c | 6 +++---
    > > > > > 1 files changed, 3 insertions(+), 3 deletions(-)
    > > > > >
    > > > > > diff --git a/arch/arm/plat-omap/iovmm.c
    > > > > b/arch/arm/plat-omap/iovmm.c
    > > > > > index 24ca9c4..fc6b109 100644
    > > > > > --- a/arch/arm/plat-omap/iovmm.c
    > > > > > +++ b/arch/arm/plat-omap/iovmm.c
    > > > > > @@ -289,19 +289,19 @@ static struct iovm_struct
    > > > > *alloc_iovm_area(struct iommu *obj, u32 da,
    > > > > > prev_end = 0;
    > > > > > list_for_each_entry(tmp, &obj->mmap, list) {
    > > > > >
    > > > > > - if (prev_end >= start)
    > > > > > + if (prev_end > start)
    > > > > > break;
    > > > > >
    > > > > > if (start + bytes <= tmp->da_start)
    > > > > > goto found;
    > > > > >
    > > > > > if (flags & IOVMF_DA_ANON)
    > > > > > - start = roundup(tmp->da_end +
    > > 1, alignement);
    > > > > > + start = roundup(tmp->da_end,
    > > alignement);
    > > > >
    > > > > There's a lack of comment here, but the purpose of
    > > > > tmp->da_end + 1 is to create a gap between iovm areas to
    > > > > force to trigger iommu faults when some access exceeds a
    > > valid area.
    > > > > Without this gap, such situation may produce data
    > > corruption which
    > > > > is much more difficult to track.
    > > >
    > > > That only works when you are accessing sequencially beyond
    > > the End of
    > > > the vm_area. However if you are accessing a random address
    > > Which is in
    > > > the mmu tables you still can corrupt memory which does Not
    > > belong to
    > > > you. That looks not very effective then why waste Memory?
    > >
    > > The main intention is to detect sequential access beyond the
    > > end of the vm area and it is effective for that purpose.
    > > i.e., OMAP3 ISP has a hw issue which makes its H3A submodule,
    > > responsible to produce statistics data for the captured
    > > image, to write more data than it should. The workaround
    > > described in the errata wasn't enough to avoid error
    > > conditions, so a different approach was implemented. This gap
    > > did help me to make sure the new workaround is valid and no
    > > data corruption was occurring anymore.
    > > Anyway, I can't see why memory is being wasted.
    > >
    >
    > I was taking about vitual memory waste (maybe not so important).
    > Is ok for me then keep the gap. Do other changes look good to
    > You?

    Do you mean in this patch?
    All changes make sense only if you're removing the gap, except for the
    fix below.

    [snip]

    > > > > >
    > > > > > prev_end = tmp->da_end;
    > > > > > }
    > > > > >
    > > > > > - if ((start > prev_end) && (ULONG_MAX - start >= bytes))
    > > > > > + if ((start >= prev_end) && (ULONG_MAX - start +
    > > 1 >= bytes))

    This fix is partially valid. The correct change must be only:
    - if ((start > prev_end) && (ULONG_MAX - start >= bytes))
    + if ((start > prev_end) && (ULONG_MAX - start + 1 >= bytes))

    Otherwise you wouldn't guarantee the gap for fixed da.

    Br,

    David


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