From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 16/22] virtio_pci: use separate notification offsets for
	each vq.
Date: Tue, 02 Apr 2013 15:51:56 -0700
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: virtualization@lists.linux-foundation.org, "Michael S. Tsirkin" <mst@redhat.com>
List-Id: virtualization@lists.linuxfoundation.org

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.

	-hpa