On Wed, Apr 03, 2013 at 04:40:29PM +1030, Rusty Russell wrote:
> "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.
Or even
addr = dev->notify_base + (vq->notify_off << dev->notify_shift);
since notify_base is per capability, shift can be per capability too.
And for IO we can allow it to be 32 to mean "always use base".
This is a bit more elegant than just saying "no offsets for IO".
--
MST
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