On 03/29/2013 08:19 PM, Rusty Russell wrote:
>>
>> Shift count?
>
> You can only have 2^16 vqs per device. Is it verboten to write 16-bit
> values to odd offsets? If so, we've just dropped it to 2^15 before you
> have to do some decoding to do. Hard to care...
>
> I dislike saying "multiply offset by 2" because implementations will get
> it wrong. That's because 0 will work either way, and that's going to be
> the common case.
>
The main reason to use a shift count is that it lets the guest driver
assume that the spacing is a power of two, requiring only shift, as
opposed to an arbitrary number, requiring a multiply. It seems unlikely
that there would be a legitimate reason for a non-power-of-two spacing
between the VQ notifiers.
The other reason is that if a particular host implementation needs
separate pages for each notifier, that can be a pretty large number.
-hpa
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