From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 16/22] virtio_pci: use separate notification offsets for each vq. Date: Wed, 3 Apr 2013 14:22:17 +0300 Message-ID: <20130403112216.GB19122@redhat.com> References: <87wqt0du2e.fsf@rustcorp.com.au> <20130324201910.GA31631@redhat.com> <8738vjer43.fsf@rustcorp.com.au> <20130326193911.GA19251@redhat.com> <87ip4d4sef.fsf@rustcorp.com.au> <20130327112535.GE24243@redhat.com> <5153CC2E.3090908@zytor.com> <878v554lsr.fsf@rustcorp.com.au> <515B610C.9000802@zytor.com> <8738v85elm.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <8738v85elm.fsf@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: virtualization@lists.linux-foundation.org, "H. Peter Anvin" List-Id: virtualization@lists.linuxfoundation.org On Wed, Apr 03, 2013 at 04:40:29PM +1030, Rusty Russell wrote: > "H. Peter Anvin" 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