Re: packed ring layout proposal v3

["Michael S. Tsirkin" <mst@redhat.com>]

If you do this whats the point of the id? Just use descriptor offset
like virtio 0 did.

On Sun, Oct 29, 2017 at 09:05:01AM +0000, Ilya Lesokhin wrote:
> My point was that if the driver is not required to change the IDs,
> it can initialized the ID's in all the descriptors when the ring is created
> and never write the ID field again.
> 
> A simple allocator for the IDs can solve the problem I presented but it is still more
> expensive then not doing ID allocation at all.
> 
> 
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > Sent: Wednesday, October 25, 2017 7:20 PM
> > To: Ilya Lesokhin <ilyal@mellanox.com>
> > Cc: virtio-dev@lists.oasis-open.org; virtualization@lists.linux-foundation.org
> > Subject: Re: packed ring layout proposal v3
> > 
> > On Sun, Oct 08, 2017 at 06:16:44AM +0000, Ilya Lesokhin wrote:
> > > > > -----Original Message-----
> > > > > From: virtualization-bounces@lists.linux-foundation.org
> > > > > [mailto:virtualization-bounces@lists.linux-foundation.org] On
> > > > > Behalf Of Michael S. Tsirkin
> > > > >
> > > > > This is an update from v2 version.
> > > >> ...
> > > > > When driver descriptors are chained in this way, multiple
> > > > > descriptors are treated as a part of a single transaction
> > > > > containing an optional write buffer followed by an optional read buffer.
> > > > > All descriptors in the chain must have the same ID.
> > > > >
> > >
> > > I apologize for the repost, I didn't realize I have to be a member of
> > > the virtio-dev mailing list.
> > >
> > > I'm concerned about the "same ID" requirement in chained descriptors.
> > 
> > It's there really just so we can remove the doubt about which descriptor's ID
> > should be used. My testing does not show a performance win from this, so I'm
> > fine with removing this requirement though I'd be curious to know why is it a
> > problem.
> > 
> > > Assuming out of order execution, how is the driver supposed to
> > > re-assign unique IDs to the previously chained descriptor?
> > 
> > For example, driver can have a simple allocator for the IDs.
> > 
> > 
> > > Is the driver expected to copy original IDs somewhere else before the
> > > chaining and then restore the IDs after the chain is executed?
> > >
> > > Thanks,
> > > Ilya
> > 
> > As device overwrites the ID, driver will have to write it out each time, that's true.
> > It's going to be a requirement even if descriptors on the chain do not need to
> > have the same ID.
> > 
> > --
> > MST