Re: [PULL 01/19] util/hbitmap: strict hbitmap_reset

[Eric Blake]

On 10/11/19 4:25 PM, John Snow wrote:
> From: Vladimir Sementsov-Ogievskiy <address@hidden>
>
> hbitmap_reset has an unobvious property: it rounds requested region up.
> It may provoke bugs, like in recently fixed write-blocking mode of
> mirror: user calls reset on unaligned region, not keeping in mind that
> there are possible unrelated dirty bytes, covered by rounded-up region
> and information of this unrelated "dirtiness" will be lost.
>
> Make hbitmap_reset strict: assert that arguments are aligned, allowing
> only one exception when @start + @count == hb->orig_size. It's needed
> to comfort users of hbitmap_next_dirty_area, which cares about
> hb->orig_size.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
> Message-Id: <address@hidden>
> [Maintainer edit: Max's suggestions from on-list. --js]
> Signed-off-by: John Snow <address@hidden>
> ---
>   include/qemu/hbitmap.h | 5 +++++
>   tests/test-hbitmap.c   | 2 +-
>   util/hbitmap.c         | 4 ++++
>   3 files changed, 10 insertions(+), 1 deletion(-)

> +++ b/util/hbitmap.c
> @@ -476,6 +476,10 @@ void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t 
> count)
>       /* Compute range in the last layer.  */
>       uint64_t first;
>       uint64_t last = start + count - 1;
> +    uint64_t gran = 1ULL << hb->granularity;
> +
> +    assert(!(start & (gran - 1)));
> +    assert(!(count & (gran - 1)) || (start + count == hb->orig_size));

I know I'm replying a bit late (since this is now a pull request), but 
would it be worth using the dedicated macro:

assert(QEMU_IS_ALIGNED(start, gran));
assert(QEMU_IS_ALIGNED(count, gran) || start + count == hb->orig_size);

instead of open-coding it?  (I would also drop the extra () around the 
right half of ||). If we want it, that would now be a followup patch.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org