0 should probably mean no shift; that way we explicitly prohibit odd offsets, which is a good thing, too. "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: >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". -- Sent from my mobile phone. Please excuse brevity and lack of formatting. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
