"H. Peter Anvin" <hpa@xxxxxxxxx> writes:
> 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.
Ah, sorry, we're talking across each other a bit.
Current proposal is a 16 bit 'offset' field in the queue data for each
queue, ie.
addr = dev->notify_base + vq->notify_off;
You propose a per-device 'shift' field:
addr = dev->notify_base + (vq->index << dev->notify_shift);
Which allows greater offsets, but insists on a unique offset per queue.
Might be a fair trade-off...
Cheers,
Rusty.
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