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Re: [Qemu-devel] [RESEND][PATCH] bitmap: get last word mask from nr dire
From: |
Wei Yang |
Subject: |
Re: [Qemu-devel] [RESEND][PATCH] bitmap: get last word mask from nr directly |
Date: |
Wed, 17 Jul 2019 13:46:47 +0800 |
User-agent: |
Mutt/1.9.4 (2018-02-28) |
On Wed, Jul 17, 2019 at 11:11:55AM +0800, Peter Xu wrote:
>On Wed, Jul 17, 2019 at 10:46:37AM +0800, Wei Yang wrote:
>> On Wed, Jul 17, 2019 at 09:59:10AM +0800, Peter Xu wrote:
>> >On Wed, Jul 17, 2019 at 09:29:02AM +0800, Wei Yang wrote:
>> >> The value left in nr is the number of bits for the last word, which
>> >> could be calculate the last word mask directly.
>> >
>> >Is it true even if start does not align to BITS_PER_LONG?
>> >
>>
>> Yes. Let me see how to explain this.
>>
>> When you look into the definition of BITMAP_LAST_WORD_MASK, it takes the
>> number of total bits and give the number of bits in last word. While the
>> value
>> matters for the input is the number of last word. This means the following
>> equation stands
>>
>> BITMAP_LAST_WORD_MASK(size) == BITMAP_FIRST_WORD_MASK(size % BITS_PER_LONG)
>>
>> Now let look at the calculation for nr. In each iteration, nr will be
>> truncated to be aligned to BITS_PER_LONG. So when we exit the loop, nr keeps
>> the number of bits in last word.
>>
>> So we can leverage the result, no matter the start is aligned or not.
>
>Yes, you are right.
>
>Do you have plan to write some unit tests for these functions? :)
>
>It'll be tests/test-bitmap.c. IMHO the test cases could be even more
>helpful to the QEMU project as a whole comparing to this patch to
>guarantee changes like your patch won't break.
Let me have a try. :-)
>
>At the meantime I think you can also do that to bitmap_set_atomic.
>
>Thanks,
>
>--
>Peter Xu
--
Wei Yang
Help you, Help me