Re: [Qemu-devel] [PULL 0/3] 128-bit support for the memory API

[Avi Kivity]

On 10/30/2011 04:12 PM, Anthony Liguori wrote:
> On 10/30/2011 09:02 AM, Avi Kivity wrote:
>> This somewhat controversial patchset converts internal arithmetic in the
>> memory API to 128 bits.
>>
>> It has been argued that with careful coding we can make 64-bit work as
>> well.  I don't think this is true in general - a memory router can
>> adjust
>> addresses either forwards or backwards, and some buses (PCIe) need the
>> full 64-bit space - though it's probably the case for all the
>> configurations
>> we support today.  Regardless, the need for careful coding means
>> subtle bugs,
>> which I don't want in a core API that is driven by guest supplied
>> values.
>
> The primary need for signed arithmetic is aliases, correct?

> Where do we actually make use of this in practice?   I think having
> negative address spaces is a weird aspect of the memory api and wonder
> if refactoring it away is a better solution tot he problem.
>

There is no direct use of signed arithmetic in the API (just in the
implementation).  Aliases can cause a region to move in either the
positive or negative direction, and this requires either signed
arithmetic or special casing the two directions.

Signed arithmetic is not the only motivation - overflow is another. 
Nothing prevents a user from placing a 64-bit 4k BAR at address
ffff_ffff_ffff_f000; we could move to base/limit representation, but
that will likely cause its own bugs.  Finally, we should be able to
represent both a 0-sized region and a 2^64 sized region.

-- 
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