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From: Don Slutz <dslutz@verizon.com>
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Date: Mon, 15 Jun 2015 12:15:52 -0400
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Subject: Re: [Qemu-devel] [BUGFIX][PATCH v7 1/9] vmport: The io memory
 region needs to be at least a size of 4
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To: Eric Blake <eblake@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Markus Armbruster <armbru@redhat.com>, Luiz Capitulino <lcapitulino@redhat.com>, Don Slutz <don.slutz@gmail.com>, Anthony Liguori <aliguori@amazon.com>, Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?QW5kcmVhcyBGw6RyYmVy?= <afaerber@suse.de>, Richard Henderson <rth@twiddle.net>

On 06/15/15 11:09, Eric Blake wrote:
> On 06/15/2015 07:53 AM, Don Slutz wrote:
>> On 06/12/15 18:38, Eric Blake wrote:
> 
>>>>  
>>>> +    /* Only support 1 address */
>>>> +    if (addr) {
>>>> +        return ~0U;
>>>> +    }
>>>
>>> Different answer on 32-bit platforms (there, ~0U is 0xffffffff, which
>>> then 0-extends to uint64_t rather than your desired result of
>>> 0xffffffffffffffffULL).
>>>
>>
>> This is not true:
> 
> Oh, I was confusing ~0UL (where sign extension on 32- vs 64-bit matters)
> and ~0U (which you used).
> 
>>
>>> Why can't you just 'return -1;'?
>>>
>>
>> I/O instructions on x86 are limited to 32bits max.  Also when EAX is
>> changed via inl, the high 32bits are 0.  So the correct result is ~0U
>> not -1.
> 
> Still, it might be better to write an explicit 0xffffffff or even have a
> named constant, rather than making people reason about whether ~0U
> promotes into a 64-bit value with only 32 bits set.
> 

Ok, Will switch to 0xffffffff.

   -Don Slutz