I mean no offset. "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
