> "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". > On Wed, Apr 03, 2013 at 07:10:42AM -0700, H. Peter Anvin wrote: > 0 should probably mean no shift; Sure. Note no shift is not same as "no offset". > that way we explicitly prohibit odd offsets, which is a good thing, too. Odd offsets? > -- > Sent from my mobile phone. Please excuse brevity and lack of formatting. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
