On Wed, Apr 03, 2013 at 07:35:31AM -0700, H. Peter Anvin wrote: > I mean no offset. I see. Fine with me. > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > >> "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. > > -- > Sent from my mobile phone. Please excuse brevity and lack of formatting. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
