From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 16/22] virtio_pci: use separate notification offsets for
	each vq.
Date: Wed, 03 Apr 2013 16:40:29 +1030
Message-ID: <8738v85elm.fsf@rustcorp.com.au>
References: <1363854584-25795-1-git-send-email-rusty@rustcorp.com.au>
	<1363854584-25795-17-git-send-email-rusty@rustcorp.com.au>
	<20130321101300.GA30493@redhat.com>
	<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>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <virtualization-bounces@lists.linux-foundation.org>
In-Reply-To: <515B610C.9000802@zytor.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: virtualization@lists.linux-foundation.org, "Michael S. Tsirkin" <mst@redhat.com>
List-Id: virtualization@lists.linuxfoundation.org

"H. Peter Anvin" <hpa@zytor.com> 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.