Re: [virtio-comment] comments for the virtio spec - Introduction section

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Thu, Jan 30, 2014 at 12:38:38PM +0200, Michael S. Tsirkin wrote:
>> On Wed, Jan 29, 2014 at 03:04:50PM +1030, Rusty Russell wrote:
>> > diff --git a/introduction.tex b/introduction.tex
>> > index 5d57f78..98e1e0d 100644
>> > --- a/introduction.tex
>> > +++ b/introduction.tex
>> > @@ -13,14 +13,14 @@ inter-guest communication) requires copying.
>> >  }
>> >  
>> >    Efficient: Virtio devices consist of rings of descriptors
>> > -  for input and output, which are neatly separated to avoid cache
>> > +  for both input and output, which are neatly separated to avoid cache
>> >    effects from both driver and device writing to the same cache
>> >    lines.
>> 
>> missed this one. I agree.
>
> Actually, rereading this, it's still IMHO not very clear what is separated
> from what. descriptors?
>
> I think output here means avail and input used?
> If yes then let's say so:
> +  for both input and output. Input rings are neatly separated from output rings
> +  to avoid cache
>
>
> Alternatively if we want to be more precise:
>
> Really descriptors are not in a ring at all:
> they are in the buffer. OTOH used ring is separated from
> avail ring, but they are not for input and output:
> input and output are in descriptor buffer.
> How about
>
> +  Efficient: Virtio devices consist of a memory buffer with descriptors
> +  for both input and output as well as memory rings of indices
> +  for driver-to-device and device-to-driver communication.
> +  device-to-driver rings are neatly separated from driver-to-device
> +  rings to avoid cache
>
> Though I think the shorter version is fine for introduction.

Indeed, good catch.  Hmm, how about we just replace the misleading
word separated?

  Virtio devices consist of rings of descriptors
  for both input and output, which are neatly laid out to avoid cache
  effects from both driver and device writing to the same cache
  lines.

>> I tweaked this one a bit more.
>> Can you pls tell me whether you agree with the patchset I posted?

Yep, getting to it now.

Cheers,
Rusty.