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Re: [libvirt] [PATCH] Fix initialization of current vcpus in libxl driver



On 05/24/2011 09:14 AM, Daniel Veillard wrote:
> On Mon, May 23, 2011 at 04:43:19PM -0600, Eric Blake wrote:
>> On 05/23/2011 04:26 PM, Jim Fehlig wrote:
>>>      b_info->hvm = hvm;
>>>      b_info->max_vcpus = def->maxvcpus;
>>> -    b_info->cur_vcpus = def->vcpus;
>>> +    b_info->cur_vcpus = (1 << def->vcpus) - 1;
>>
>> That's a compilation pitfall if def->vcpus is exactly 32.
>>
>> Shoot; xenFormatSxpr in xenxs/xen_sxpr.c has the same bug.
> 
> What would be the best way to fix ? cast to a larger int and
> recast the result or provide some kind of helper function for
> bitmaps ?

Maybe special-case:

if (def->vcpus == 32)
  b_info->cur_vcpus = (uint32_t) -1;
else
  b_info->cur_vcpus = (1 << def->vcpus) - 1;

Or build it up differently (although it gets harder to read):

b_info->cur_vcpus = ((1 << (def->vcpus - 1)) - 1) * 2 + 1;


As long as we rework it to avoid the undefined overflow of 1 << 32, when
we really just want 32 bits set.

-- 
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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