Re: [Qemu-devel] [PATCH v2] bitmap: fix BITMAP_LAST_WORD_MASK

[Wei Wang <wei.w.wang@intel.com>]

On 08/07/2018 05:53 PM, Dr. David Alan Gilbert wrote:
> * Wei Wang (wei.w.wang@intel.com) wrote:
>> On 08/07/2018 03:39 PM, Peter Xu wrote:
>>> On Tue, Jul 31, 2018 at 06:01:18PM +0800, Wei Wang wrote:
>>>> When "nbits = 0", which means no bits to mask, this macro is expected to
>>>> return 0, instead of 0xffffffff. This patch changes the macro to return
>>>> 0 when there is no bit needs to be masked.
>>>>
>>>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
>>>> CC: Juan Quintela <quintela@redhat.com>
>>>> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>>> CC: Peter Xu <peterx@redhat.com>
>>> Reviewed-by: Peter Xu <peterx@redhat.com>
>>>
>>> Is there any existing path that can trigger this nbits==0?
>> Not sure about other bitmap APIs which call this macro. But it happens in
>> the patches we are working on, which use bitmap_count_one.
>> It would be good to have the macro itself handle this corner case, so that
>> callers won't need to worry about that.
> Given that I see you're having a similar discussion on the kernel list
> we should see how that pans out before making qemu changes.

OK.
The situation is a little different in Linux, because all the callers 
there have already taken the responsibilities to avoid the "nbits=0" 
corner case, that's also the reason that they want to stick with the old 
way. Here in QEMU, most callers (e.g. bitmap_count_one, bitmap_fill, 
bitmap_intersects) haven't checked that, so fixing the macro itself 
might be a better choice here.

Best,
Wei